Syntactic sugar - allow easier config of @PactTestFor(providerName = "farmer", port = "8084")

[holly-cummins]

Instead of writing

@PactTestFor(providerName = "farmer", port = "8084")

it would be nice to be able to provide a rest client config key (similar to what we set in @RegisterRestClient(configKey = "farmer-api"). That would give us the URL and should also allow us to work out a sensible default provider name.

Alternatively, we pass through a class name of a RestClient, just as we do with some other Quarkus tests.

[holly-cummins]

See also #69, which is a more extreme version of this.

[holly-cummins]

This would also allow us to provide different config for %test profiles, to avoid port conflicts between mock and real servers.