OwSink.combine is derived from Akka's Sink.combine but has no Lightbend attribution

[pjfanning]

OwSink.combine in common/scala/src/main/scala/org/apache/openwhisk/core/containerpool/logging/DockerToActivationFileLogStore.scala says it was copied from Akka:

object OwSink {

  /**
   * Combines two sinks into one sink using the given strategy. The materialized value is a Tuple2 of the materialized
   * values of either sink. Code basically copied from {@code Sink.combine}
   */
  def combine[T, U, M1, M2](first: Sink[U, M1], second: Sink[U, M2])(

It was added in d69f8db (#2974, 2017), when OpenWhisk built against Akka 2.5.6. It is derived from Sink.combine in Akka 2.5.6's Sink.scala, which is Apache-2.0 and carries:

Copyright (C) 2014-2017 Lightbend Inc. <http://www.lightbend.com>

There is no licence conflict, but the derived code is not attributed. The file only has the ASF header, and neither LICENSE.txt nor NOTICE.txt mentions Akka or Lightbend. Apache-2.0 §4(c) requires keeping the copyright notices of the source.

Suggested fix, following what ConcurrentMapBackedCache.scala already does for its Spray-derived code:

1. Keep the ASF header and add the upstream notice below it:

   /*
    * OwSink.combine is derived from Sink.combine in Akka 2.5.6:
    * Copyright (C) 2014-2017 Lightbend Inc. <http://www.lightbend.com>
    */

2. Add an entry to LICENSE.txt next to the Spray one, naming the file, Akka 2.5.6, Apache-2.0 and the Lightbend copyright line.

Akka 2.5.6 ships no NOTICE file, so NOTICE.txt does not need to change. Re-deriving the code from Apache Pekko instead would not avoid this: Pekko's Sink.scala still carries the Lightbend copyright header.

Apache OpenServerless vendors this file and is fixing it in apache/openserverless-build#22 and apache/openserverless#270 (tracked in apache/openserverless#265).