Add option to allow push force in protected branches

[Pigueiras]

There is no way to specify allow_force_push when setting up the protected branches

https://docs.gitlab.com/ee/api/protected_branches.html#protect-repository-branches

[gdubicki]

I think that we should start passing raw parameters for protected branches.

If we want to keep the backward compatibility (= not release v3 already) we can do it when we detect push_access_level or merge_access_level or unprotect_access_level set.

Then we would have this feature future-proof like project settings is.

I would be happy to accept a PR with this. :)

[gdubicki]

Oops, actually this is not yet implemented.. 😅