Add method SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]

Just like it is very useful to use `DataStream[T]` as a sort of Functor or Monad with `map`/`flatMap`/`filter` methods, it would be extremely handy to have a `SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]` on `SinkFunctions`, so that you can reuse existing complex sink functions, but with a different input type. For example:

val bucketingStringSink: SinkFunction[String] = 
  new BucketingSink[String]("...")
    .setBucketer(new DateTimeBucketer("yyyy-MM-dd-HHmm")

val bucketingIntListSink: SinkFunction[List[Int]] =
  bucketingStringSink.contramap[List[Int]](_.mkString(","))

For some more formal motivation behind this, https://typelevel.org/cats/typeclasses/contravariant.html is definitely a great place to start!