[2.2][WIP] Integrates metadata into Symfony validator

A starting point for #2992 feedback appreciated and necessary. Currently only the xml driver have been ported together with the very basic implementation where a ClassMetadata is gotten.

Still needs the following:

* MethodMetadata
* PropertyMetadata
* Implement metadata caching

Note that no files have been removed yet, that will happen when its more complete, and works as a replacement.