Backport `disable_joins` to enable usage of cross-DB relations

As part of identified changes of PoC is to backport support for disable_joins. disable_joins allows to mark passthrough relations as having to use pluck instead of cross-joins. This is essential if the relation defines a model living in another DB.

class Runner
   has_many :projects, through: :runner_projects, disable_joins: true
end

Related:

• Initial backport: https://gitlab.com/gitlab-org/gitlab/-/blob/6284efc3560b0d7d6aa216e0de0a2e2f154b89e0/config/initializers/00_rails_disable_joins.rb
• Initial backport: https://gitlab.com/gitlab-org/gitlab/-/blob/6284efc3560b0d7d6aa216e0de0a2e2f154b89e0/config/initializers/00_rails_disable_joins_on_joins.rb
• https://github.com/rails/rails/pull/41937
• https://github.com/rails/rails/pull/42079
• https://github.blog/2021-07-12-adding-support-cross-cluster-associations-rails-7/

Proposal

1. Backport disable_joins in a vanila way: 1-to-1 change
2. Since usage of disable_joins might have negative impact we need have a way to dynamically toggle the feature to validate this on production
3. Add additional (temporary feature) on top of disable joins to allow this to by feature-flagged (this should be done on top of backport, the best as an additional mixin). We should remove it as soon as possible as no longer needed.

class Runner
   # allow to feature flag disable join usage to rollout it to the production as it might
   has_many :projects, through: :runner_projects, disable_joins: -> { Feature.enabled?(:ci_runner_projects_disable_joins) }
end