Expose rehydration via grpc

The rehydration logic was implemented in the transaction manager in !7948 (merged).

This MR adds support for detecting and triggering repository rehydration via the OptimizeRepository RPC:

• Introduces GetConfigValues and IsOffloaded helpers to localrepo for reading and interpreting offload configuration.
• Adds a new STRATEGY_REHYDRATION enum to the OptimizeRepositoryRequest.
• Wires the rehydration logic into the OptimizeRepository RPC, allowing offloaded repositories to be rehydrated on demand.

Closes #6610 (closed)