Custom mapping support

[apenlor]

As a developer, I want to fully customize how a data type is processed.

For this to be available, we could extend builder object methods to overwrite the DSL definition generated.

For example: Having a Student object that contains an Example object, besides having available the setExample we can add something like `setExampleDslPart'

        //already available
        StudentBuilder studentBuilder = new StudentBuilder();
        studentBuilder.setExample(new Example( ... ))
        DslPart student = studentBuilder.build();

        import static au.com.dius.pact.consumer.dsl.LambdaDsl.newJsonBody;

         DslPart dslPartForExample = newJsonBody(object -> {
            object.stringType("id", "1");
            object.stringType("name", "Fake name");
            object.stringType("email", "some.email@sngular.com");
            object.numberType("studentNumber", 23);
        }).build();

        StudentBuilder studentBuilder = new StudentBuilder();
        //to be added
        studentBuilder.setExampleDslPart(dslPartForExample)
        DslPart student = studentBuilder.build();

Something like this would open the possibility of fully customizing how an object is managed by the library.

Also would be good to have a way to configure this behavior at the annotation level, to avoid having to repeat this assignation in each one of the '@Pact' definitions. Maybe use a new annotation that references an implementation class.

For example:

@PactDslBodyBuilder
public class Student {

  @CustomMapping(CustomMapping.class)
  private Example example;

}

That referenced class should contain an annotated method that references Example.class, to indicate that is the DslPart generator method:

public class CustomMapping {

  @CustomMappingGenerator(Example.class)
  public Example getExampleDslPart (){
         return newJsonBody(object -> {
            object.stringType("id", "1");
            object.stringType("name", "Fake name");
            object.stringType("email", "some.email@sngular.com");
            object.numberType("studentNumber", 23);
        }).build();
 }

}