perf: reduce number of queries made by /api/v2/workspaceagents/{id} (#21522)

Relates to https://github.com/coder/internal/issues/1214

The `ExtractWorkspaceAgentParam` middleware ends up making 4 database
queries to follow the chain of `WorkspaceAgent` -> `WorkspaceResource`
-> `ProvisionerJob` -> `WorkspaceBuild` -- but then dropping all that
hard work on the floor. The `api.workspaceAgent` handler that references
this middleware then has to do all of that work again, plus one more
query to get the related `User` so we can get the username. This pattern
is also mirrored in `getDatabaseTerminal` but without the middleware.

This PR:
* Adds a new query `GetWorkspaceAgentAndWorkspaceByID` to fetch all
this information at once to avoid the multiple round-trips,
* Updates the existing usage of `GetWorkspaceAgentByID` to this new
query instead,
* Updates `ExtractWorkspaceAgentParam` to also store the workspace in
the request context

Dalibo: [0.63ms](https://explain.dalibo.com/plan/40bb597f3539gc6c)

coderd/httpmw/workspaceagentparam_test.go

@@ -86,7 +86,7 @@ func TestWorkspaceAgentParam(t *testing.T) {
 		db, _ := dbtestutil.NewDB(t)
 		dbtestutil.DisableForeignKeysAndTriggers(t, db)
 		rtr := chi.NewRouter()
-		rtr.Use(httpmw.ExtractWorkspaceAgentParam(db))
+		rtr.Use(httpmw.ExtractWorkspaceAgentAndWorkspaceParam(db))
 		rtr.Get("/", nil)
 
 		r, _ := setupAuthentication(db)
@@ -113,10 +113,10 @@ func TestWorkspaceAgentParam(t *testing.T) {
 				RedirectToLogin: false,
 			}),
 			// Only fail authz in this middleware
-			httpmw.ExtractWorkspaceAgentParam(dbFail),
+			httpmw.ExtractWorkspaceAgentAndWorkspaceParam(dbFail),
 		)
 		rtr.Get("/", func(rw http.ResponseWriter, r *http.Request) {
-			_ = httpmw.WorkspaceAgentParam(r)
+			_ = httpmw.WorkspaceAgentAndWorkspaceParam(r)
 			rw.WriteHeader(http.StatusOK)
 		})
 
@@ -140,10 +140,10 @@ func TestWorkspaceAgentParam(t *testing.T) {
 				DB:              db,
 				RedirectToLogin: false,
 			}),
-			httpmw.ExtractWorkspaceAgentParam(db),
+			httpmw.ExtractWorkspaceAgentAndWorkspaceParam(db),
 		)
 		rtr.Get("/", func(rw http.ResponseWriter, r *http.Request) {
-			_ = httpmw.WorkspaceAgentParam(r)
+			_ = httpmw.WorkspaceAgentAndWorkspaceParam(r)
 			rw.WriteHeader(http.StatusOK)
 		})