Is your feature request related to a problem? Please describe (👍 if you like this request)

Need a gRPC service to serve as a portal for the operations requested by the Longhorn manager on each node. It will also be responsible for managing the SPDK tgt process to serve all functions supported by SPDK JSON RPC calls from the control plane.

Describe the solution you'd like

Create a gRPC service, and call SPDK manager or similar for replica management, logical volume store management, etc. The code can be in longhorn-spdk-engine then used by instance manager.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

cc @longhorn/qa