Implement config management

[kate-osborn]

Parent ticket: #292

Implement the configuration management piece of the control plane. The control plane should send the nginx configuration to all registered agents over the Commander Service's Download channel. The control plane should handle any errors and retry if possible.

A/C:
For every change in Kubernetes that requires an nginx update, the control plane must:
- generates an nginx config payload containing all relevant configs and files.
- send a command to all registered agents to Download the config
- chunk out and send the nginx config payload over the Download channel to all agents
- report success/failure of config apply
- on error, retry if possible

Aha! Link: https://nginx.aha.io/features/NKG-37