Group-level protected branches - REST API

Problem description

As described in the parent epic &8679 Users want to be able to have standardized workflows across teams and projects and leverage protected branches as a way to do this.

Implementation plan

1. Add the attribute inherited to the project-level protected branches API. (Have no this in the group-level one)
2. Update ProtectedBranchesFinder to support the group-level.
3. Add the REST API of group-level protected branches.
4. Update API docs.
5. Update API specs.