Resolve additional repository's replica path on creation calls

The object pool service RPCs carry an additional repository along with the target repository. This repository lies on the same storage as the target repository. The additional repository's relative path needs to also be rewritten by Praefect with the replica path as it would have been when the additional repository was originally created.

Praefect currently does so for other mutator RPCs than 'create' type as there are no 'create' type RPCs yet that contain an additional repository. CreateObjectPool is incorrectly not marked as 'create' type call though. In preparation for fixing that, this commit already introduces relative path rewriting for additional repositories in 'create' type RPCs.

Part of: #4030 (closed)