Implement agent registration

[kate-osborn]

Parent ticket: #292

The agent will register with the control plane on startup. Write a control plane component that handles agent registration. This component is responsible for the following:

• registering a new connection from an agent
• storing agent connections alongside agent info (agent ID, nginx IDs, host info, tags, etc)
• disconnecting with an agent

A/C:

• Multiple agents can register with the control plane
• Control plane can track the connection status for each agent
• Control plane can disconnect and reconnect with an agent
• Log new agent connections/disconnections. Include total number of registered agents in the log messages.

Aha! Link: https://nginx.aha.io/features/NKG-36

[pleshakov]

for troubleshooting / debugging - both for users and developers, would it make sense to expose logs and/or metrics that count connected agents?

[kate-osborn]

for troubleshooting / debugging - both for users and developers, would it make sense to expose logs and/or metrics that count connected agents?

Yep, that makes sense. We don't expose any metrics ATM so let's do logs.