Add API for reconstructing descriptors and generic signatures

[Ladicek]

This API includes type variable substitution, which is used on several places in Quarkus. If the substitution function returns a type for some type variable identifier, the reconstructed descriptor/signature contains a descriptor/signature of the provided type instead of the original type variable.

[Ladicek]

Draft for now, as I need to test if the changes here are sufficient for Quarkus usage of the type variable substitution feature.