Skip protected branches permission check for fork sync actions

Part of epic &16678

Problem

Introduce `allow_fork_sync` field to database (#517139) adds a new field allow_fork_sync to protected_branches table.

We want to skip the permission check when fork sync toggle is enabled for the branch.

Proposal

Use Gitaly context support introduced in !178463 (merged).

Pass gitaly_context: { fork_sync: true } to user_merge_branch and user_ff_branch when a merge action triggered by Projects::Forks::SyncService.

Skip permission check when the allow_fork_sync: true for the branch

See spike: !176718 (f184977c)