feat: add group protected branches service

Prakash Divyrequested to merge
prakashdivyy/client-go:feat/implement-group-protected-branches into main

This commit adds a new service for interacting with the Group Protected Branches API. It includes support for listing, getting, protecting, unprotecting, and updating group-level protected branches.

Key changes:

  • Added GroupProtectedBranchesService in group_protected_branches.go
  • Initialized service in gitlab.go
  • Added unit tests in group_protected_branches_test.go
  • Reused ProtectedBranch structs from protected_branches.go
  • Updated generated mocks

Relates to #2147

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