feat: add codersdk constructor that uses an independent transport (#22282)

This is useful at least in the case of scaletests but potentially in
other places as well. I noticed that scaletest workspace creation
hammers a single coderd replica.
---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

scaletest/createworkspaces/run.go

@@ -77,7 +77,14 @@ func (r *Runner) Run(ctx context.Context, id string, logs io.Writer) error {
 			return xerrors.Errorf("create user: %w", err)
 		}
 		user = newUser.User
-		client = codersdk.New(r.client.URL)
+		// Duplicate the client with an independent transport to ensure each
+		// workspace creation gets its own HTTP connection pool. This prevents
+		// HTTP/2 connection multiplexing from causing all workspace GET requests
+		// to route to a single backend pod during load testing.
+		client, err = loadtestutil.DupClientCopyingHeaders(r.client, nil)
+		if err != nil {
+			return xerrors.Errorf("duplicate client: %w", err)
+		}
 		client.SetSessionToken(newUser.SessionToken)
 	}