Group push rules: execute read operations from group_push_rules table

Implementation plan

We want to execute read operations using the new group_push_rules table instead of push_rules. To achieve this, we need to perform the following changes:

• Control the feature flow using a feature flag (:group_push_rules). If the FF is on then we use group_push_rules, if the FF is off then we use push_rules.
• Filter GroupPushRule by group param :id in the GET group_push_rule API endpoint https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/api/group_push_rule.rb#L69.
• We might need to replace the association push_rule that points to push_rules table (source).

belongs_to :push_rule, inverse_of: :group

With something like (untested code) that points to group_push_rules table:

belongs_to :group_push_rule, inverse_of: :group

• Change the predefined_push_rule method in ee/app/models/ee/group.rb to read from :group_push_rules if the FF is on.

Expectations

• The GET request for group_push_rules should return a GroupPushRule record without changes in the API response.
• Use the group_push_rule association in the group model if the FF is on.
• Create the rollout and cleanup issues for the FF (if it has not been created yet).