jandex binding work

A page to collect proposals, todos and follow-up work in regards to the ongoing (long-term) jandex-binding work.

Unified mapping (XML) schema

https://hibernate.atlassian.net/browse/HHH-8893

Essentially:

• Provide an "extended" orm.xml schema which adds Hibernate-specific definitions "on top of" the JPA orm.xml schema
• Deprecate hbm.xml source and temporarily provide transformation (both on-the-fly and build-time) of hbm.xml to this new schema