With https://gitlab.com/gitlab-org/gitlab/-/merge_requests/150420,
partitioning hints will be provided by Rails via the gRPC request
metadata when invoking the ReplicateRepository RPC. This is done during
project moves to ensure Gitaly partitions the repository and its object
pools in the same partition.

Modify the stream director so it attempts to extract the partitioning
hint from the incoming context. This is treated the same way as an
additional repository, so it gets rewritten to the @cluster/... style
path that Gitaly cluster expects. Finally, ensure the rewritten path is
set back into the request context which is later used by the partition
manager.

Modify the coordinator's ReplicateRepository tests to exercise the
partitioning hint branch too.