Add get_buffered_amount() to WebRTCDataChannel

[dsnopek]

This methods gives access to RTCDataChannel.bufferedAmount - see:

https://developer.mozilla.org/en-US/docs/Web/API/RTCDataChannel/bufferedAmount

This can be useful for checking if SCTP is buffering data (possibly due to its flow control kicking in) and then waiting to send more data until the buffer clears.

This comment on DataChannelInterface::Send() is a really good explanation of the motivation for this:

  // Sends |data| to the remote peer. If the data can't be sent at the SCTP
  // level (due to congestion control), it's buffered at the data channel level,
  // up to a maximum of 16MB. If Send is called while this buffer is full, the
  // data channel will be closed abruptly.
  //
  // So, it's important to use buffered_amount() and OnBufferedAmountChange to
  // ensure the data channel is used efficiently but without filling this
  // buffer.
  virtual bool Send(const DataBuffer& buffer) = 0;

https://github.com/maitrungduc1410/webrtc/blob/master/api/data_channel_interface.h#L191

This won't cleanly cherry-pick to 3.x and requires some minor changes, so I'll make a separate PR.

I'll also make a PR to the GDNative plugin, but I'm not sure how the GDNative part will work with backward compatibility? This changes the godot_net_webrtc_data_channel struct which will cause old version of the GDNative plugin to not work until updated, and also updated versions of the GDNative plugin won't work with older versions of Godot.

[Faless]

Looks good, great job! 🏆

[Faless]

Thanks!