Implement mock host

[sachalevy]

Issues such as #10 require the client to post a request and have it processed by a host running in the background. Beyond mocking the HTTP request, and response, we need to implement a mock host capable of handling the request and returning a result to the client. Note that this does not need to be implemented as an actual host running in a background thread, but could be designed as a method to be called by the HTTP mock framework to run the Hugging Face pipeline and retrieve results.

The mock host should be made available through methods contained in a util file, in the test folder.