Add admin_push_rules custom ability

What does this MR do and why?

• Adds admin_push_rules as an ability for custom roles.
• Custom roles created with admin_push_rules ability, on top of any base role, will allow users to manage push rules settings for repositories in groups and projects
• This MR just introduces the ability. The policy changes will be made in future MRs.

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

How to set up and validate locally

1. Enable the custom_ability_admin_push_rules Feature Flag:

   Feature.enable(:custom_ability_admin_push_rules)

2. Log-in as an owner of a group and navigate to Settings > Roles and Permissions

3. You should be able to create a custom role with Admin push rules checkbox enabled.

Related to #421786 (closed)