Implement `agent` injection routine.

[DanielleMaywood]

The agent should have a routine listening for newly created dev containers.

Once a new dev container is detected, it should:

1. Communicate with coderd to create a workspace_agents record and receive an auth_token.
2. Detect the architecture of the dev container
3. Check if a copy of an agent with that architecture is stored, otherwise download a new one.
4. Copy that binary onto the agent.
5. Attempt to add CAP_NET_ADMIN to the injected dev-container binary.
6. Run the coder agent command in the dev container using the auth_token obtained earlier.