Merge using extension

[velo]

Proposal for addition for merge using syntax related to querydsl/querydsl#2520
Few notes:

• This changes interfaces, japicmp-maven-plugin will complain
• Merge using query works also with latest versions of H2 and PostgreSQL, however bumping versions results in other tests breaking
• In case of Oracle it currently produces Caused by: java.sql.SQLSyntaxErrorException: ORA-00969: missing ON keyword. Reason being it doesn't put "as" between subquery and its alias, not sure how to fix nicely.

Copy of querydsl/querydsl#3625

[velo]

@borgiiri I will hold merging this till I hear you ok with it

[borgiiri]

Thanks, if you want you can also merge this as is and I can provide updates as separate PR.
As of now the merge using syntax does work if you use queryFactory.merge(...).using(SQLExpressions.select(...).from(...).as(...)).on(...)...
I'm quite busy in the coming days, so will only manage to continue this properly next week.

[velo]

Sweet, will merge as is then