Add client context handler to inject correlaction id into client context

Currently, the correlation_id does not get propagated all the way into GitLab's internal API, making it hard to debug operations that involve the internal API. This series of commits injects the correlation id into the Gitaly Client Context, which already does get passed along to the internal API.

NOTE: A rails side change will be needed to log this value

fixes: #6680 (closed)