Allow git_push_options to be passed to UserRebaseConfirmable RPC

Related issue: gitlab#38141 (closed)

The goal of this MR is to allow the rebase operation from Rails side to send ci.skip push option and for Gitaly to pass it through down to post-receive hook, which in turn is consumed by Rails.

In order to do that I've:

• modified UserRebaseConfirmableRequest.Header to accept a git_push_options field (repeatable)
• introduced a PushOptions class that would be responsible for handling push options (specifically converting header.git_push_options flag to push option environment vars)
• modified Gitlab::Git::Hook to take in input PushOptions object and set env variables when running the hook