chore(cdk): add Application tag to all stacks (#97)

## Summary
- Add `Application: ServerlessWebappStarterKit` tag to the entire CDK
app
- Uses `cdk.Tags.of(app).add()` to automatically propagate the tag to
all resources across both stacks

Author: badmintoncryer

cdk/bin/cdk.ts

@@ -52,6 +52,8 @@ new MainStack(app, 'ServerlessWebappStarterKitStack', {
   signPayloadHandler: virginia.signPayloadHandler,
 });
 
+cdk.Tags.of(app).add('Application', 'ServerlessFullStackWebappStarterKit');
+
 // import { Aspects } from 'aws-cdk-lib';
 // import { AwsSolutionsChecks } from 'cdk-nag';
 // Aspects.of(app).add(new AwsSolutionsChecks());