Propagate client identity to gitaly

This is the client-side counterpart to gitaly!2802 (merged).

Propagating the user_id and username will make it easier to tell where user traffic is coming from. It makes gitaly ever so slightly user aware.

The remote_ip is unfortunately the HAProxy one for operations over SSH. So it will not be very useful yet. That is being tracked in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10954 (see also #165 (closed) and !394 (merged)).

Note: The longer-term goal is to put this logic into labkit: labkit!93. Once that lands, we can switch this code over.