Change extension unit tests to mock HttpClient

[Oceanswave]

Unit tests of the extensions project are a bit flaky as they depend on uptime of httpbin, which is good, but sometimes can throttle or have invalid responses.

Steps:

• Inject HTTP Client as a singleton via DI in both the fetch implementation and the webresource middleware.
• Use mockhttp in unit tests - https://github.com/richardszalay/mockhttp