* Move Streams.copy into elasticsearch-core and make a multi-release jar
This moves the method `Streams.copy(InputStream in, OutputStream out)` into the
`elasticsearch-core` project (inside the `o.e.core.internal.io` package). It
also makes this class into a multi-release class where the Java 9 equivalent
uses `InputStream#transferTo`.
This is a followup from
https://github.com/elastic/elasticsearch/pull/29300#discussion_r178147495