praefect: Introduce `GetObjectPoolHandler()`

The GetObjectPool() RPC should return object pool information that can be consumed by the client. When this Gitaly RPC is executed behind Praefect, the replica path is returned instead of the relative path. The replica path is an implementation detail of Praefect and not useful to the client which really wants the relative path.

This change implements a GetObjectPoolHandler() to explicitly handle the GetObjectPool() RPC in Praefect instead of relying on the UnknownServiceHandler to proxy the RPC. This enables the RPC's response replica path to be reversed by Praefect to the appropriate relative path and storage the client expects.