R1:d94d1da61098

Proxy Diffusion Conduit API calls

Summary:
Fixes T7020. When an external user makes a Conduit request to Diffusion but the repository isn't hosted locally, we need to proxy it.

This also adds a guard layer to prevent requests from getting infinitely proxied inside the cluster.

In "trivial" configurations (where the repository is a service repository, but the service is on the local device) I'm making us always proxy anyway. This basically makes it reasonable to test this stuff (otherwise you'd have to…