WebHook testing example to HelloWorld repo, also WebSocket

### Background
- Recently in twitter a folk from CapGemini is looking for WebHook testing.
- Tweet link: https://twitter.com/iamsoham/status/1090977861404631040
- AT the face of it sounds like set of Http calls
- Investigate if any further complexities involved
- We can simply override the `HttpClient` to deal with WebHooks. Or even without it?

### TODO in this ticket-
- Add some HelloWorld examples to showcase the WebHook testing
- Add some WebSocket examples to showcase the WebSocket testing

#### _References-_
- The server side push event testing demo could help here if any async calls involved
- @BeTheCodeWithYou can throw some light here or point to the examples as he has achieved this
- Overriding HttpClient exmple can be linked here if that helps e.g. OAuth2 by @santhoshTpixler

_Tweet has some details which sounds like achievable straigh away, but provide examples._

> @sohamda