Handle CreateObjectPool as a repository creation RPC

CreateObjectPool creates the object pool but Praefect does not handle it as a repository creating RPC. This causes subsequent operation on the pool repository to fail as Praefect doesn't create the relevant metadata records. This commit marks the RPC as a repository creating RPC.

With the metadata correctly created, the object pool service's test now work with Praefect again without the metadata creation hack. Object pools still face additional problems on Gitaly Cluster. There's no special handling from them, so it can be that the pool repository's primary differs from the main repository's. This may cause problems as the pool operations are not correctly replicated to the secondaries. In combination with variable replication factor, it can be that the pool repository ends up on a different node entirely than the main repository.

Closes: #4030 (closed)
Related to: #4019 (closed)