Implement `coderd` side of RPC communication

[DanielleMaywood]

We need the parent agent to be able to create/delete/list dev containers from coderd. The agent.proto definition should be updated and implemented on the coderd side.

message CreateDevContainerAgentRequest {
	string name = 1;
	string directory = 2;
}

message CreateDevContainerAgentResponse {
	bytes id = 1;
	bytes auth_token = 2;
}

message DeleteDevContainerAgentRequest {
  bytes id = 1;
}

message ListDevContainerAgentsResponse {
	message DevContainerAgent {
		string name = 1;
		bytes id = 2;
	}

	repeated DevContainerAgent agents = 1;
}

service Agent {
	...
  rpc CreateDevContainerAgent(CreateDevContainerAgentRequest) returns (CreateDevContainerAgentResponse);
  rpc DeleteDevContainerAgent(DeleteDevContainerAgentRequest) returns ();
  rpc ListDevContainerAgents() returns (ListDevContainerAgentsResponse);
}