Implement backend events for stubbing/spying WebSockets

[flotwig]

Build out backend events to support stubbing/spying WebSockets for #199

• Add support to add:route for matching on WebSockets
• Match incoming websocket connections against registered net-stubbing routes
• Add events to communicate WebSocket info between driver and server and to support interception of frames/connects/disconnects. Draft spec:

eventName	from	to	payload	description
ws:connect	server	driver	CyIncomingRequest, CyWebSocket	fired when a new WS connection is received
ws:connect:continue	driver	server	CyIncomingRequest	fired when driver wishes to pass the connection on to destination
ws:disconnect	server	driver	CyWebSocket	fired when a WS disconnects
ws:frame:outgoing	server	driver	CyWebSocket, CyWebSocketMessage	fired when browser sends a ws frame
ws:frame:outgoing:continue	driver	server	CyWebSocketMessage	continue passing frame to upstream
ws:frame:incoming	server	driver	CyWebSocket, CyWebSocketMessage	fired when upstream server sends a ws frame
ws:frame:incoming:continue	driver	server	CyWebSocketMessage	continue passing frame to browser

Depends on #687

[unlikelyzero]

This would be a game-changer for test automation. I can't wait for this to land! Let me know if you'd like me to test a preview PR!

[deepanshuverma]

any update on this one guys, when can we expect this ?

[cypress-app-bot]

This issue has not had any activity in 180 days. Cypress evolves quickly and the reported behavior should be tested on the latest version of Cypress to verify the behavior is still occurring. It will be closed in 14 days if no updates are provided.

[Andarist]

this feature would be awesome, bump for a stale bot

[suppadeliux]

Would be awesome to support this!

[jove4015]

Would be so great to have this! We are waiting on data from a websocket to refresh and right now we have to go through hurdles to update hidden dom elements on the page so that we can notify cypress that the data is done loading.... it's beyond kludgy. Having a "cy.intercept" that works with websocket messages would be so helpful.