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/agentapi/connectionlog_test.go

@@ -114,10 +114,9 @@ func TestConnectionLog(t *testing.T) {
 			api := &agentapi.ConnLogAPI{
 				ConnectionLogger: asAtomicPointer[connectionlog.ConnectionLogger](connLogger),
 				Database:         mDB,
-				AgentFn: func(context.Context) (database.WorkspaceAgent, error) {
-					return agent, nil
-				},
-				Workspace: &agentapi.CachedWorkspaceFields{},
+				AgentID:          agent.ID,
+				AgentName:        agent.Name,
+				Workspace:        &agentapi.CachedWorkspaceFields{},
 			}
 			api.ReportConnection(context.Background(), &agentproto.ReportConnectionRequest{
 				Connection: &agentproto.Connection{