This commit makes it configurable which metadata will be run
by the Node when starting the Restate server.

This commit adds the skeleton of the Raft metadata store. At the moment
only a single node with memory storage is supported.

This fixes restatedev#1785.

The raft metadata store does not accept new proposals if there is
no known leader. In this situation, request failed with an internal
ProposalDropped error. This commit changes the behavior so that a
ProposalDropped error will be translated into an unavailable Tonic
status. That way, the request will get automatically retried.

This commit adds RocksDbStorage which implements raft::Storage.
The RocksDbStorage is a durable storage implementation which is
used by the RaftMetadataStore to store the raft state durably.

This fixes restatedev#1791.

This fixes restatedev#1803.