Introduce `allow_fork_sync` field to database
Part of epic https://gitlab.com/groups/gitlab-org/-/epics/16678
### Problem
We need to keep the state of the fork sync toggle for protected branches.
### Proposal
```
add_column :protected_branches, :allow_fork_sync, :boolean, default: false, null: false
```
See spike: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/176718/diffs#48d25e31b47ed892d556034c8d3e4ef57a42d36e_0_10
issue