Draft: Add `gitaly_squash_using_merge` feature flag

When (and if) gitaly!4514 (merged) will be merged, we will need to add new feature flag.

I'm adding feature flag definition gitaly_squash_using_merge that corresponds to squash_using_merge added in gitaly!4514 (merged).

I'm marking this MR as a draft, as that other MR has not been merged yet.