Support "enhanced DynamoDB client in the AWS SDK for Java v2"

[nyh]

In April 2020, Amazon introduced yet another variant of their DynamoDB SDK, the "enhanced DynamoDB client in AWS SDK for Java v2" (https://aws.amazon.com/blogs/developer/introducing-enhanced-dynamodb-client-in-the-aws-sdk-for-java-v2/) ".

It isn't clear how to use this API together with our load balancer. Our java/pom.xml contains the line:

TODO: test also dynamodb-enhanced for v2: https://aws.amazon.com/blogs/developer/introducing-enhanced-dynamodb-client-in-the-aws-sdk-for-java-v2/

Which I added when I removed a dynamodb-enhanced artifact from the POM and wasn't sure what to do instead.

As the comment suggests, I think we should create in java/src/test/java/com/scylladb/alternator/test a fourth Java demo (we already have one v1, one v2, and one v2 with async) which uses the "enhanced client" and then we need to check what needs to be done to make it work with load balancing.