Refactored class and method generation (#1116)

following two event storming sessions with @pilloPl and @OlgaMaciaszek and the Spring Cloud Contract community we've decided to refactor the core modules of Spring Cloud Contract Verifier.

The main idea is that the current `JavaTestGenerator`'s implementation no longer uses complex logic to generate classes and methods.

We've introduced abstractions such as:

-    class body builder (to build classes)
-    groovy and java class metadata
-    imports (for various frameworks)
-    default setup for json, xml, restassured, jaxrs
-    class annotations for spock, junit and junit5
-    field insertion e.g. for messaging

docs/src/main/asciidoc/verifier_contract.adoc

@@ -1800,7 +1800,7 @@ following code listing shows the `SingleTestGenerator` interface:
 
 [source,groovy]
 ----
-include::{verifier_core_path}/src/main/groovy/org/springframework/cloud/contract/verifier/builder/SingleTestGenerator.groovy[indent=0,lines=17..-1]
+include::{verifier_core_path}/src/main/groovy/org/springframework/cloud/contract/verifier/builder/SingleTestGenerator.java[indent=0,lines=17..-1]
 ----
 
 Again, you must provide a `spring.factories` file, such as the one shown in the following