Allow running Wiremock without HTTP Server

[jamietanna]

As noted in #1476, a common search online is to run Wiremock without
needing an HTTP server, for instance if using a serverless deployment
model.

It is currently possible to do this using the WireMockApp, which
relies on internal APIs, which will break when we move to Java Modules.

To avoid this, we can implement the DirectCallHttpServer as a first
class citizen, which, when created, will allow consumers to retrieve the
mapped Response (whatever it may be) for Stub and Admin requests.

We can use the existing Jetty stopTimeout configuration to control the
configuration for this server's async timeouts, so consumers can
override it in cases they need delays on responses.

Because we're using the HttpResponders as callbacks, we need to use
Mockito's doAnswer to invoke the real HttpResponder from our mock.

We also add the Mockito Jupiter bindings to more easily write JUnit
Jupiter tests using Mockito.

Finally, we want to add an example test to show consumers how they can
use it for themselves.

Closes #1476.

---

Depends on #1565, but otherwise ready for reviews

• What did we want to do docs-wise?
• This doesn't handle the chunked delay distributions
• This doesn't handle Faults

[tomakehurst]

Hey, please could you rebase this against master?

[jamietanna]

Thanks Tom, have done 👍

[timtebeek]

Suggested change

	assertDoesNotThrow(() -> server.start());
	assertDoesNotThrow(() -> server.stop());