Implement WebSocket server skeleton for external integrations

[bdach]

• Supersedes / closes Broadcast game state through web sockets #18129. Reasons I didn't use that PR are hopefully obvious upon comparing diffs but I can elaborate if they are not.
• Single metric included for demonstration purposes.
• Do not want to talk about further schema design at this time.
• Specify OSU_WEBSOCKET_SERVER=1 envvar to enable.
• Can test consumption with this five minute html job (works even as a standalone file opened in browser, no CORS bs!)
• There's a lot of inline comments, go read them. There are many WTFs because the .NET frozen websocket API is weird and stanky and reeks of the year 2007. The inline comments attempt to explain.