Add remaining MR approval settings to group level approval settings API

Why are we doing this work

To support the management of all MR approval settings at group level, we need to expose some additional attributes to the group level approval settings API.

Relevant links

Related issue that introduced the initial group MR settings table and fields

Non-functional requirements

• [-] Documentation: Not needed as it is behind a feature flag
• Feature flag: group_merge_request_approval_settings_feature_flag
• [-] Performance:
• [-] Testing:

Implementation plan

• database Add these new fields to the group_merge_request_approval_settings table
  • allow_committer_approval
  • allow_overrides_to_approver_list_per_merge_request
  • retain_approvals_on_push
  • require_password_to_approve

• backend Add a new field to the API entity of Group MR approval settings (verify with both GET and PUT)