Adds feature flag for GetCommitSignatures which
got ported to go.

More info: https://gitlab.com/gitlab-org/gitaly/merge_requests/1283

The feature flag has been introduced an was turned off by default,
now the it will default to be turned on. That change would still allow
users to turn this feature off by leveraging the Rails console by
running:

`Feature.disable("gitaly_catfile-cache")`

Another option is to manage the number of items the LRU cache will
contain, by updating the `config.toml` for Gitaly. This would be the
`catfile_cache_size`:
https://gitlab.com/gitlab-org/gitaly/blob/0dcb5c579e63754f557aef91a4fa7a00e5b8b127/config.toml.example#L27

Closes: https://gitlab.com/gitlab-org/gitaly/issues/1712

The GitalyClient held a lot of logic which was all very tightly coupled.
In this instance the feature logic was extracted to make it do just a
little less and create a bit more focus in the GitalyClient's
responsibilies.