Implement coderd <> Agent communication

[defelmnq]

Description

Devcontainers will rely on agents running on each devcontainer and will be connected to the parent agent - running on the workspace.
An advantage of this solution is that we will be able to have more visibility on the devcontainer lifecycle - and so on when to refresh the coder_app applications.

Flow

devcontainer_agent --contact on start--> coderd
coderd --request associated coder_app--> workspace_agent
workspace_agent --returns apps from devcontainers.json--> coderd

[mafredri]

Closing this as coderd/agent communications has been implemented for sub agent creation, display apps, and apps. (Related #614)