adds NIOWebSocketFrameAggregator to buffer fragmented WS frames

[dnadoba]

Motivation:

We want support frame fragmentation at the WebSocket level

Modifications:

add NIOWebSocketFrameAggregator to channel pipeline in front of our own channel handlers

Result:

Fragmented WebSocket Frames are now supported.

Discussion

What are good default values for minNonFinalFragmentSize and maxAccumulatedFrameCount for WebSocket?
The equivalent of minNonFinalFragmentSize in RSocket would be 64 bytes but I don't know if this is appropriate for WebSocket as well.
I have set minNonFinalFragmentSize by default to 0 and maxAccumulatedFrameCount to Int.max for now.

minNonFinalFragmentSize and maxAccumulatedFrameCount are part of WSTransport and not ClientConfiguration because they are specific to WebSocket. I kind of want them to be part of ClientConfiguration. I initially thought that we could just add these properties to ClientConfiguration.Fragmentation and even use the same values for WebSocket fragmentation and RSocket fragmentation. But that would mean one can not configure fragmentation separately for WebSocket and for RSocket... Maybe that's good thing, less configuration one has to worry about is in general a good thing. However, I can't say for sure that RSocket fragmentation and WebSocket fragmentation should always have the same buffer limits.

[OlegDokuka]

LGTM