The named data source part for https://github.com/quarkusio/quarkus/issues/46727

I know we [discussed doing named PU together](https://github.com/quarkusio/quarkus/issues/46727#issuecomment-2765814753), but this works already and the changes seems easy to understand so I wanted to try create the smaller one first.

[this one was supposed to be done together as well](https://github.com/quarkusio/quarkus/issues/47034) but it doesn't seem to be related to named-datasource so I assume it's related to named persistent units.