Scala 3 support

[pjfanning]

• we have a real mixed bag of connectors in this project
• some are easy to make work with Scala3
• others have dependencies that don't support Scala3 or where the dependency version we use is old and there may be compatibility issues with upgrading
• some cases involve getting the code to compile in Scala3 - some looks pretty tricky - other code is easy to get working

We may be able to get some connectors Scala 3 ready for pekko connectors 1.0.0.

Currently PRs are being merged to the scala3 branch (not main).

See https://github.com/apache/incubator-pekko-connectors/labels/scala3 for other issues - where work is delayed till later.

[nvollmar]

Getting the ones we easily can might already be enough for the other downstream projects to continue.

[pjfanning]

@nvollmar #123 gets about half of them working but doesn't cover some of the most popular ones like the cassandra one.