Implement gRPC service handler for Private Scratchpad operations #28

[traktion]

Resolves #28

Implemented gRPC service handler for Private Scratchpad operations:

• Created proto/private_scratchpad.proto with gRPC service and message definitions.
• Created src/grpc/private_scratchpad_handler.rs implementing the gRPC service.
• Integrated the new handler into src/lib.rs.
• Updated src/service/scratchpad_service.rs to allow field access and cloning for the gRPC handler.
• Added unit tests for proto mapping.
• Incremented patch version in Cargo.toml.