net/quinn: Add support for WebTransport

This MR adds support for WebTransport, a new web API that allows for low-level, bidirectional communication between a client and a server, available in the browser as an alternative to HTTP and WebSockets.

The following 2 new elements have been created:

• quinnwtserversink: Acts as a WebTransport server allowing to stream to a WebTransport client (the client can be a browser or a GStreamer app)
• quinnwtclientsrc: Acts as a WebTransport client and it can be used to test the server.

The implementation uses the web_transport_quinn crate, which in turn uses the quinn crate. For this reason, we decided to include the new elements in the quinn plugin.

Design choices:

• The sink element always acts as a WebTransport server.
• The sink element blocks in start until a new connection is made.
• When working with streams instead of datagrams, the server creates the connection, which is bi-directional to support upstream events in the future.
• A secure connection is always required.

Usage:

gst-launch-1.0 quinnwtclientsrc url="https://localhost:4445" certificate-file=net/quinn/tests/certs/localhost.crt caps=audio/x-opus ! opusparse ! opusdec ! audioconvert ! autovideosink

gst-launch-1.0 audiotestsrc is-live=true ! opusenc ! quinnwtserversink address=127.0.0.1 port=4445 certificate-file=net/quinn/tests/certs/localhost.crt private-key-file=net/quinn/tests/certs/localhost.key 

Missing features

The following features are missing:

• allowPolling: the server only supports one session per connection.
• requireUnreliable: there is no fallback to establish a connection over HTTP/2
• congestionControl: congestion control can't be configured

This work was started during the GStreamer hackfest and has been co-authored with @rgonzalez.

cc: @SanchayanMaity