Add gRPC server and skeleton for Commander Service

[kate-osborn]

Parent ticket: #292

Add a gRPC server to the control plane and a skeleton implementation of the gRPC Commander Service.

In order to keep PRs for this feature small, this task does not cover implementing the logic of agent registration and configuration management. That will be handled in a separate task. The purpose of this to add the plumbing for the Commander Service and verify that the agent and control plane can communicate with each other.

A/C:

• Agent can connect to the control plane's gRPC server. Verified by log messages.
• Agent can send and receive commands via the Commander Service's CommandChannel. Verified by log messages.

Aha! Link: https://nginx.aha.io/features/NKG-35