feat: Implement RaftInspectionService, RaftManipulationService skeleton

[jopemachine]

Defines the skeleton for useful gRPC calls that allow administrators to look inside the cluster.
I believe these gRPC calls can make cluster debugging and testing easier.

• It'd be better if these APIs need to be reviewed.
• If the API list is not needed, it can be easily removed by excluding x-api from the feature.
• Implementing each method could be "Good first issue" for newcomers
• Most methods can be implemented as sort of wrapper of RawNode's methods.
• People who are not familiar with Rust can also test with Python.