Rehydrating gRPC endpoint implementation

With !7948 (merged), the housekeeping logic of rehydration is implemented.

In this issue, we want to expose this logic by a the optimize_repository gRPC endpoint.

Exposing by a gRPC endpoint means we need to do rehydration manually. This is not ideal. The ultimate goal of rehydration is put it in a gPRC middleware, so that any write request will trigger a rehydration so that we don't need to manually do that.