introduce struct libwebsocket_extension

lib/client-handshake.c

lib/libwebsockets.c

lib/libwebsockets.h

lib/private-libwebsockets.h

libwebsockets-api-doc.html

test-server/test-client.c

test-server/test-ping.c

test-server/test-server-extpoll.c

test-server/test-server.c