Add ability to reference Maintainers or Developers from CODEOWNERS

What does this MR do and why?

Add the ability to reference any default role to a code owners file using an @@ prefix. See the attached screen grab for the result.

Local migration output

Up

main: == [advisory_lock_connection] object_id: 128900, pg_backend_pid: 62084
main: == 20241002130409 AddRoleApproversToApprovalMergeRequestRules: migrating ======
main: -- add_column(:approval_merge_request_rules, :role_approvers, :integer, {:array=>true, :default=>[], :null=>false})
main:    -> 0.0388s
main: == 20241002130409 AddRoleApproversToApprovalMergeRequestRules: migrated (0.0428s)

main: == [advisory_lock_connection] object_id: 128900, pg_backend_pid: 62084

Down

main: == [advisory_lock_connection] object_id: 128900, pg_backend_pid: 60837
main: == 20241002130409 AddRoleApproversToApprovalMergeRequestRules: reverting ======
main: -- remove_column(:approval_merge_request_rules, :role_approvers)
main:    -> 0.0024s
main: == 20241002130409 AddRoleApproversToApprovalMergeRequestRules: reverted (0.0077s)

main: == [advisory_lock_connection] object_id: 128900, pg_backend_pid: 60837

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Reference_developers_from_codeowners_file_small

How to set up and validate locally

1. Enable the codeowner_role_approvers feature flag.
2. Setup a local project with codeowners.
3. In the codeowners file specify a file and a role. eg: README.md @@developer
4. Make an MR which changes the specified file and notice the list of recommended approvers are developers in the project.

Related to #282438
Spike issue #479928 (closed)