feat: Add agent authentication based on instance ID (#336)

* feat: Add agent authentication based on instance ID

Each cloud has it's own unique instance identity signatures, which
can be used for zero-token authentication. This change adds support
for tracking by "instance_id", and automatically authenticating
with Google Cloud.

* Add test for CLI

* Fix workspace agent request name

* Fix race with adding to wait group

* Fix name of instance identity token

cli/clitest/clitest_test.go

@@ -18,7 +18,7 @@ func TestMain(m *testing.M) {
 func TestCli(t *testing.T) {
 	t.Parallel()
 	clitest.CreateProjectVersionSource(t, nil)
-	client := coderdtest.New(t)
+	client := coderdtest.New(t, nil)
 	cmd, config := clitest.New(t)
 	clitest.SetupConfig(t, client, config)
 	pty := ptytest.New(t)