WebSocket: allow UpgradeToWebSocket to support flows of FrameEvents

[pjfanning]

We removed the deprecated WebSocketUpgrade class in 2.0.0-M1.

Play needs access to work at a low level and just the Message level that WebSocketUpgrade supports. (playframework/playframework#13744)

pekko-http/http-core/src/main/scala/org/apache/pekko/http/impl/engine/ws/UpgradeToWebSocketLowLevel.scala

Line 43 in 75c2e90

private[http] def handleFrames(handlerFlow: Graph[FlowShape[FrameEvent, FrameEvent], Any],

As you can see in UpgradeToWebSocketLowLevel, it should be easy enough expose FrameEvent support.

I prefer this to adding back the WebSocketUpgrade code and supporting both approaches again.

If we do change UpgradeToWebSocket support maybe we can hide the frame event support and have users cast their UpgradeToWebSocket instance to UpgradeToWebSocketLowLevel to access the additional handleFrames method.

Edit
Play already has a hack to access UpgradeToWebSocketLowLevel so we may get away with not changing anything in Pekko. They have a org.apache.pekko.http class in Play to skip around the package private setting on UpgradeToWebSocketLowLevel.