Extreme syntactic sugar - allow tests to use same model as we do for rest clients

[holly-cummins]

When we define a rest client mock, the suggested pattern is this:

    @InjectMock
    @RestClient
    Client mock;

    @BeforeEach
    public void setUp() {
        when(mock.get()).thenReturn("MockAnswer");
    }

It would be nice to be able to do something similar for Pact, something like:

    @InjectPactMock
    @RestClient
    Client contractMock;

    @BeforeEach
    public void setUp() {
        when(contractMock.get()).thenReturn("MockAnswer");
    }

(Or perhaps it would still be in a @Pact block, but as concise.)

The current pattern is much more verbose, and almost all of the content (ports, content type headers, endpoints, schema for return and input parameters ... ) is in there is already in the RestClient definition:

    @Pact(provider = "farmer", consumer = "knitter")
    public V4Pact createPact(PactDslWithProvider builder) throws JsonProcessingException {
        Map<String, String> headers = new HashMap<>();
        headers.put("Content-Type", "application/json");

        // Here we define our mock, which is also our expectations for the provider
        String sweaterBody = "{\"colour\":\"white\"}\n";
        return builder.given("test POST")
                .uponReceiving("POST REQUEST")
                .path("/wool/order")
                .method("POST")
                .willRespondWith()
                .status(200)
                .headers(headers)
                .body(sweaterBody)
                .toPact(V4Pact.class);
    }

Things that aren't in the client that would need to be defined in the equivalent of when(contractMock.get()).thenReturn("MockAnswer") are the various states (given), and the response values.