Add support for TREAT operator

[beikov]

JPA 2.1 introduced the TREAT operator and we should add support for that in the parser and the API.

[beikov]

Some issues that we should keep in mind

https://hibernate.atlassian.net/browse/HHH-9345
https://hibernate.atlassian.net/browse/HHH-9342
https://hibernate.atlassian.net/browse/HHH-8974

[beikov]

Treat is allowed to be used as

• join path expression
• normal path expression

We have to reflect that in the grammar