Add methods to reconstruct generic signature

[Ladicek]

Classes, methods and fields (and record components) may have generic signature in the bytecode. Jandex parses it and then throws away. It is sometimes useful to reconstruct the signature, possibly with applying type variable substitutions. Quarkus does that on several occasions and I think it would be nice if it was in Jandex proper.

It is also important to be able to tell whether a generic signature is required for given class/method/field. Sometimes, you want to reconstruct the signature unconditionally, while sometimes, you only want to reconstruct the signature when required.