Add WebRTC support using libdatachannel

The WebRTC is exposed via `/video.html` endpoint
and enabled by default as long as h264 stream
is available.

.vscode/c_cpp_properties.json

@@ -4,6 +4,8 @@
             "name": "Linux",
             "includePath": [
                 "${workspaceFolder}/**",
+                "${workspaceFolder}/third_party/libdatachannel/include",
+                "${workspaceFolder}/third_party/libdatachannel/deps/json/include",
                 "/usr/include/libcamera",
                 "/usr/include/liveMedia",
                 "/usr/include/groupsock",
@@ -13,7 +15,8 @@
             "defines": [
                 "USE_LIBCAMERA=1",
                 "USE_FFMPEG=1",
-                "USE_RTSP=1"
+                "USE_RTSP=1",
+                "USE_LIBDATACHANNEL=1"
             ],
             "compilerPath": "/usr/bin/gcc",
             "cStandard": "gnu17",