feat: add javadsl api for the Migration Tool

[sebastian-alfers]

Currently the Migration Tool only has a scala api which requires the java user to convert the results (scala futures), which is not very convenient.

It was recommended to introduce a scaladsl and javadsl package, and then to move the existing scala based implementation one to scaladsl packed and then create a subclass (or facade) where it currently resides with the same name, and deprecate that.

Constructor signature should be maintained to stay binary compatible, if possible.