Iterators: support transform() with JDK Function class

[reschke]

Currently:

https://guava.dev/releases/snapshot-jre/api/docs/com/google/common/collect/Iterators.html#transform(java.util.Iterator,com.google.common.base.Function)

...would be good to relax that to

transform(java.util.Iterator,java.util.function.Function)

so it becomes easier to migrate away from Guava Function class.