Implements count() for named queries

[loicmathieu]

Fixes #10319

[loicmathieu]

@FroMage to implement a count() for a named query I had to retrieve the query string. The easiest way I find is by creating the named query and casting it to org.hibernate.query.Query.

As the code is alreadt couple to Hibernate I don't think it's an issue but maybe you know a better way to do this.

I could have processed the @NamedQuery annotation at build time to create a cache of queries, but I found this overkill, as count is cached inside PanacheQuery, this code don't have to be performance efficient.

[FroMage]

LGTM

[FroMage]

Yeah, this works fine enough, thanks a lot!