feat: Add Sink#count operator.

[He-Pin]

Motivation:
refs: #2182

[Copilot]

Pull Request Overview

This PR adds a new count operator to the Sink API that counts all incoming elements until upstream terminates, providing a convenient way to get the total element count from a stream.

Key changes:

• Adds Sink.count operator to both Scala and Java APIs
• Implements the counting functionality via a new CountSink GraphStage
• Includes comprehensive test coverage and documentation

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
stream/src/main/scala/org/apache/pekko/stream/scaladsl/Sink.scala	Adds Scala API for Sink.count with documentation
stream/src/main/scala/org/apache/pekko/stream/javadsl/Sink.scala	Adds Java API for Sink.count with documentation
stream/src/main/scala/org/apache/pekko/stream/impl/fusing/CountSink.scala	Implements the counting logic as a GraphStage
stream/src/main/scala/org/apache/pekko/stream/impl/Stages.scala	Adds default attributes for the count sink
stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/SinkSpec.scala	Adds Scala test for the count operator
stream-tests/src/test/java/org/apache/pekko/stream/javadsl/SinkTest.java	Adds Java test for the count operator
docs/src/test/java/jdocs/stream/operators/SinkDocExamples.java	Adds Java documentation example
docs/src/main/paradox/stream/operators/index.md	Updates operator index to include count
docs/src/main/paradox/stream/operators/Sink/count.md	Adds complete documentation for the count operator

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[mdedetrich]

@He-Pin I added an extra commit for a more performant solution to the java.lang.Long/scala.lang.Long issue. While it is true that scala.long.Long is a supertype of java.lang.Long, because CompletionStage is invariant it doesn't compile as is so you have to use asInstanceOf.

The Java SinkTest is passing locally without any issues, but ill wait till tomorrow morning (my time) for all the tests to pass so I can approve the PR.

[mdedetrich]

lgtm