fix: limit calls to GetWorkspaceAgentByID in agentapi (#23015)

We currently call GetWorkspaceAgentByID millions of times at scale
unnecessarily. This PR embeds immutable fields into the relevant
services instead of fetching for them every time.

resolves https://github.com/coder/scaletest/issues/84

Confirmed with a 10k scaletest that this changeset takes the query from
10M+ queries down to 39k

coderd/workspaceagentsrpc.go

@@ -178,7 +178,7 @@ func (api *API) workspaceAgentRPC(rw http.ResponseWriter, r *http.Request) {
 
 		// Optional:
 		UpdateAgentMetricsFn: api.UpdateAgentMetrics,
-	}, workspace)
+	}, workspace, workspaceAgent)
 
 	streamID := tailnet.StreamID{
 		Name: fmt.Sprintf("%s-%s-%s", workspace.OwnerUsername, workspace.Name, workspaceAgent.Name),