feat: secure and cross-domain subdomain-based proxying (#4136)

Co-authored-by: Kyle Carberry <kyle@carberry.com>

coderd/coderdtest/authorize_test.go

@@ -10,6 +10,8 @@ import (
 func TestAuthorizeAllEndpoints(t *testing.T) {
 	t.Parallel()
 	client, _, api := coderdtest.NewWithAPI(t, &coderdtest.Options{
+		// Required for any subdomain-based proxy tests to pass.
+		AppHostname:              "test.coder.com",
 		Authorizer:               &coderdtest.RecordingAuthorizer{},
 		IncludeProvisionerDaemon: true,
 	})