Directly expose WebRTCPeerConnection::create_data_channel() to GDExtension

[dsnopek]

Due to godot-cpp bug godotengine/godot-cpp#652, rather than directly exposing WebRTCPeerConnection::create_data_channel(), an alternate version which returned an Object * (instead of Ref<T>) was exposed, with a custom create_data_channel() that called it.

Since that bug is now fixed, we can remove the workaround, and directly expose it!

I think this breaks compatibility, but since we're thoroughly breaking GDExtension compatibility for Godot 4.1 anyway, this seems like a good time to change this. :-)

[dsnopek]

Here's a corresponding PR for webrtc-native to adjust to these changes: godotengine/webrtc-native#101

I tested webrtc-native and this PR using the webrtc_minimal and webrtc_signaling demos in https://github.com/godotengine/godot-demo-projects - it's working great for me!

[RedworkDE]

Validate extension JSON: Error: Field 'classes/WebRTCPeerConnectionExtension/methods/_create_data_channel/return_value': type changed value in new API, from "Object" to "WebRTCDataChannel". needs to be added to misc/extension_api_validation/4.0-stable.expected

[dsnopek]

@RedworkDE Thanks! Should be fixed in my latest push

[akien-mga]

Thanks!