Preload group root ancestor for Group Projects API

Describe in detail what your merge request does and why.

Related to #330140 (closed)

Feature flag

While this change should be relatively safe, it would have significant impact if it did cause issues. For that reason I added a feature flag which we can quickly remove if there are no issues in production.

Feature flag rollout issue: #354372 (closed)

Resolves a few related N+1s in the group projects API:

• Preloads the group relation for projects, after namespace has been preloaded. For all projects except those in a user namespace, group and namespace is identical so it doesn't make sense to request two queries to get the same record.
• Preloads root_ancestor for namespace and group. Several places in this request path require root_ancestor including the resolution of the default branch name, and some permission policy conditions.

Queries for which N+1's are now eliminated

D, [2022-02-28T14:22:28.375193 #843] DEBUG -- :   CACHE Namespace Load (0.0ms)  SELECT "namespaces"."id", "namespaces"."name", "namespaces"."path", "namespaces"."owner_id", "namespaces"."created_at", "namespaces"."updated_at", "namespaces"."type", "namespaces"."description", "namespaces"."avatar", "namespaces"."membership_lock", "namespaces"."share_with_group_lock", "namespaces"."visibility_level", "namespaces"."request_access_enabled", "namespaces"."ldap_sync_status", "namespaces"."ldap_sync_error", "namespaces"."ldap_sync_last_update_at", "namespaces"."ldap_sync_last_successful_update_at", "namespaces"."ldap_sync_last_sync_at", "namespaces"."description_html", "namespaces"."lfs_enabled", "namespaces"."parent_id", "namespaces"."shared_runners_minutes_limit", "namespaces"."repository_size_limit", "namespaces"."require_two_factor_authentication", "namespaces"."two_factor_grace_period", "namespaces"."cached_markdown_version", "namespaces"."project_creation_level", "namespaces"."runners_token", "namespaces"."file_template_project_id", "namespaces"."saml_discovery_token", "namespaces"."runners_token_encrypted", "namespaces"."custom_project_templates_group_id", "namespaces"."auto_devops_enabled", "namespaces"."extra_shared_runners_minutes_limit", "namespaces"."last_ci_minutes_notification_at", "namespaces"."last_ci_minutes_usage_notification_level", "namespaces"."subgroup_creation_level", "namespaces"."emails_disabled", "namespaces"."max_pages_size", "namespaces"."max_artifacts_size", "namespaces"."mentions_disabled", "namespaces"."default_branch_protection", "namespaces"."unlock_membership_to_ldap", "namespaces"."max_personal_access_token_lifetime", "namespaces"."push_rule_id", "namespaces"."shared_runners_enabled", "namespaces"."allow_descendants_override_disabled_shared_runners", "namespaces"."traversal_ids" FROM "namespaces" WHERE "namespaces"."id" = 132 LIMIT 1
D, [2022-02-28T14:22:28.375832 #843] DEBUG -- :   ↳ app/models/namespaces/traversal/linear.rb:103:in `block in root_ancestor'
D, [2022-02-28T14:22:28.375845 #843] DEBUG -- :   ↳ lib/gitlab/utils/strong_memoize.rb:28:in `strong_memoize'
D, [2022-02-28T14:22:28.375852 #843] DEBUG -- :   ↳ app/models/namespaces/traversal/linear.rb:99:in `root_ancestor'
D, [2022-02-28T14:22:28.375858 #843] DEBUG -- :   ↳ app/models/concerns/has_repository.rb:86:in `default_branch_from_group_preferences'
D, [2022-02-28T14:22:28.375864 #843] DEBUG -- :   ↳ app/models/concerns/has_repository.rb:79:in `default_branch_from_preferences'
D, [2022-02-28T14:22:28.375870 #843] DEBUG -- :   ↳ app/models/concerns/has_repository.rb:75:in `default_branch'
D, [2022-02-28T14:22:28.375875 #843] DEBUG -- :   ↳ app/models/project.rb:2673:in `default_branch_or_main'
D, [2022-02-28T14:22:28.375881 #843] DEBUG -- :   ↳ lib/gitlab/json.rb:110:in `dump'
D, [2022-02-28T14:22:28.375887 #843] DEBUG -- :   ↳ lib/gitlab/json.rb:110:in `adapter_dump'
D, [2022-02-28T14:22:28.375892 #843] DEBUG -- :   ↳ lib/gitlab/json.rb:42:in `dump'

D, [2022-02-28T13:49:45.398313 #89721] DEBUG -- :   CACHE Namespace Load (0.0ms)  SELECT "namespaces"."id", "namespaces"."name", "namespaces"."path", "namespaces"."owner_id", "namespaces"."created_at", "namespaces"."updated_at", "namespaces"."type", "namespaces"."description", "namespaces"."avatar", "namespaces"."membership_lock", "namespaces"."share_with_group_lock", "namespaces"."visibility_level", "namespaces"."request_access_enabled", "namespaces"."ldap_sync_status", "namespaces"."ldap_sync_error", "namespaces"."ldap_sync_last_update_at", "namespaces"."ldap_sync_last_successful_update_at", "namespaces"."ldap_sync_last_sync_at", "namespaces"."description_html", "namespaces"."lfs_enabled", "namespaces"."parent_id", "namespaces"."shared_runners_minutes_limit", "namespaces"."repository_size_limit", "namespaces"."require_two_factor_authentication", "namespaces"."two_factor_grace_period", "namespaces"."cached_markdown_version", "namespaces"."project_creation_level", "namespaces"."runners_token", "namespaces"."file_template_project_id", "namespaces"."saml_discovery_token", "namespaces"."runners_token_encrypted", "namespaces"."custom_project_templates_group_id", "namespaces"."auto_devops_enabled", "namespaces"."extra_shared_runners_minutes_limit", "namespaces"."last_ci_minutes_notification_at", "namespaces"."last_ci_minutes_usage_notification_level", "namespaces"."subgroup_creation_level", "namespaces"."emails_disabled", "namespaces"."max_pages_size", "namespaces"."max_artifacts_size", "namespaces"."mentions_disabled", "namespaces"."default_branch_protection", "namespaces"."unlock_membership_to_ldap", "namespaces"."max_personal_access_token_lifetime", "namespaces"."push_rule_id", "namespaces"."shared_runners_enabled", "namespaces"."allow_descendants_override_disabled_shared_runners", "namespaces"."traversal_ids" FROM "namespaces" WHERE "namespaces"."id" = 132 LIMIT 1
D, [2022-02-28T13:49:45.399008 #89721] DEBUG -- :   ↳ app/models/namespaces/traversal/linear.rb:103:in `block in root_ancestor'
D, [2022-02-28T13:49:45.399019 #89721] DEBUG -- :   ↳ lib/gitlab/utils/strong_memoize.rb:28:in `strong_memoize'
D, [2022-02-28T13:49:45.399025 #89721] DEBUG -- :   ↳ app/models/namespaces/traversal/linear.rb:99:in `root_ancestor'
D, [2022-02-28T13:49:45.399032 #89721] DEBUG -- :   ↳ app/models/project.rb:2634:in `root_namespace'
D, [2022-02-28T13:49:45.288326 #89721] DEBUG -- :   ↳ ee/app/policies/ee/project_policy.rb:130:in `block (2 levels) in <module:ProjectPolicy>'
D, [2022-02-28T13:49:45.288331 #89721] DEBUG -- :   ↳ app/models/ability.rb:79:in `allowed?'
D, [2022-02-28T13:49:45.288337 #89721] DEBUG -- :   ↳ lib/api/entities/project.rb:59:in `block in <class:Project>'
D, [2022-02-28T13:49:45.288342 #89721] DEBUG -- :   ↳ lib/gitlab/json.rb:110:in `dump'
D, [2022-02-28T13:49:45.288356 #89721] DEBUG -- :   ↳ lib/gitlab/json.rb:110:in `adapter_dump'
D, [2022-02-28T13:49:45.288361 #89721] DEBUG -- :   ↳ lib/gitlab/json.rb:42:in `dump'

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