[Database] Support "nulls not distinct" PostgreSQL unique constraints

[thierry2015]

I currently don't see an easy way to run the add constraint {name} unique nulls not distinct statement in database migrations when using PostgreSQL >= 15 (supported since this was introduced). There's an unanswered Laracast question on that topic as well.

I'd like to be able to add $table->unique($myColumns)->nullsNotDistinct() when building a database migration, which could easily be applied by the Postgres schema grammar (I have a working example on an existing project where I extended the original grammar, more than happy to take care of the implementation in the Lravel framework)

[thierry2015]

#55025