When we call Gitaly and set feature flags, we can't easily guarantee
that we can get the current project (if there even is such a thing).
However, there are some cases where we do have easy access to a project,
if one exists: when we're performing Git HTTP or SSH operations.

This allows - but doesn't require - Gitaly feature flags to be set for a
project, as long as the call site supports it. In this commit, that
means the two cases above (Git HTTP and SSH).