deps: Add websocketpp as vendored code

Make it easier to apply patches as official releases are not openssl4
compatible, yet.

deps/websocketpp/tutorials/broadcast_tutorial/broadcast_tutorial.md

@@ -0,0 +1,17 @@
+Broadcast Tutorial
+==================
+
+This tutorial will dig into some more nitty gritty details on how to build high
+scalability, high performance websocket servers for broadcast like workflows.
+
+Will go into features like:
+- minimizing work done in handlers
+- using asio thread pool mode
+- teaming multiple endpoints
+- setting accept queue depth
+- tuning compile time buffer sizes
+- prepared messages
+- flow control
+- basic operating system level tuning, particularly increasing file descriptor limits.
+- measuring performance with wsperf / autobahn
+- tuning permessage-deflate compression settings