Support Wiremock API for generating json stubs

[otrosien]

At the moment the stub json is fixed by introspecting the request. Instead it should be possible to use and manipulate the wiremock stub in the test case.

[otrosien]

see spring cloud contract for an example.

[otrosien]

The easiest way would be to provide call-backs so that the initial stub mappings can be customized.

com.github.tomakehurst.wiremock.stubbing.StubMapping.buildFrom(mappingDefinition);

see https://github.com/spring-cloud/spring-cloud-contract/blob/master/spring-cloud-contract-wiremock/src/main/java/org/springframework/cloud/contract/wiremock/WireMockStubMapping.java

[otrosien]

So we'll end-up in: Map<String,Object> -> JSON String -> StubMapping -> callback to caller -> modified StubMapping -> JSON String.