Add Group protected branches API

What does this MR do and why?

Related to #389442 (closed), epic &8679.

Previous MR: !90040 (merged), !106532 (merged)

• Add the API and docs of the group protected branch

Roadmap

1. Add groups support to ProtectedBranchesFinder (!107988 - merged)
2. Add the `inherited` field to Protected Branches (!110602 - merged)
3. 👉 Add Group protected branches API (!110603 - merged)

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:group_protected_branches)

2. The REST API of group-level protected branches will work as expected like the project-level one.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc EM @daveliu