Added a Spring-based consumer

* Goal: re-stage the contract with Spring for educational purposes
* Generates a totally different contract for now...
* Is copied from the code-example for the PactDslJsonBody DSL
* See: https://docs.pact.io/implementation_guides/jvm/consumer#building-json-bodies-with-pactdsljsonbody-dsl[example-code]
* Added Maven-Builddirectory to .gitignore
* Added the generated JSON for comparison