Treated path operation support and testing

[jwgmeligmeyling]

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[jwgmeligmeyling]

Turns out that we didn't need any special handling for treat operations, as these are embedded in the serialised join expression and accepted by the Blaze-Persistence JPQL Next parser.

I did find out though that QueryDSL only supported TREAT operations in the FROM clause. JPA also describes the possibility to use TREAT operations in the WHERE clause using treated paths. Some JPA providers even support TREAT in SELECT, ORDER BY, etc. Blaze-Persistence may rewrite TREAT to JOINS in unsupported cases (Blazebit/blaze-persistence#409).

It turned out to be rather difficult to implement TREAT support in the builder API, because the implementation of Path and PathImpl is rather closed for extension. It however was relatively easy to fix in QueryDSL itself. For this pull request querydsl/querydsl#2530 has been created.