Provide in-memory implementations of HttpServletResponse/Request

[dzikoysk]

A fully working implementation of req/res could be an option to all Javalin users that cannot simply start/stop whole instances and that use mocks these days.

The implementation should be relatively simple, most of our API goes through Context interface that implements most of the API with default functions.

We could provide something like:

// given: a request
val context = TestContextSpec()
  .body("abc")
  .header(HttpHeaders.AUTHORIZATION, "Basic xyz")

// when: endpoint is called
endpoint.handle(context.build())

// then: it's ok
assertThat(context.status()).isEqualTo(HttpStatus.OK)