!str #15063 Add Flow.Transformer interface

* Skip wrapping RecoveryTransformer elements in Try
* this will make it possible to use the same RecoveryTransformer interface
  for javadsl and scaladsl, which is important if we package building
  blocks in Transformer classes
* also avoids wrapper allocation
Merge pull request #15066 from akka/wip-15063-transformer-patriknw

!str #15063 Add Flow.Transformer interface