Validate identity generated id for CockroachDB

[DavideD]

Fix #1338

The error message now looks like:

org.hibernate.HibernateException: HR000073: java.lang.Short is an invalid identity type when using CockroachDB (entity org.hibernate.reactive.IdentityGeneratorTypeForCockroachDBTest$ShortTypeEntity) - CockroachDB might generates identifiers that are too big and won't always fit in a java.lang.Short. java.lang.Long is valid replacement

I don't think this was a critical issue before because we threw exceptions based on the size of the identifiers.
But this should give users more info about what's going on.

It would be nicer to validate this at startup

[gavinking]

I would prefer a shorter message, but otherwise it looks fine.

[blafond]

Ran locally fine. good changes to asserting error message content.

Looks good.

[DavideD]

Thanks