Admin page: add back output snapshot endpoint

[jenshu]

We used to show the Proxy CRs in the output snapshot. Those have been replaced by GatewayIR.

• Determine if we still need this endpoint to return the IRs, or if the xds snapshot endpoint is sufficient
• If we want an endpoint that returns the GatewayIRs, figure out the best way to retrieve that info (they are not currently stored in a krt collection)