Properly handle data channel opening failures

[tomaka]

Fix #2900

In order to open outbound substreams on a WebRTC connection, the mechanism that the API of smoldot uses is as follows: one function to ask the implementation to open a substream, and one callback that is later called in order to notify that a substream has been fully opened.

The API of smoldot doesn't provide a way to report that opening an outbound substream has failed. This is because this isn't really supposed to happen. If you have a connection, opening a substream is purely a "local side change": you allocate an ID, then send a message. It should maybe not even be asynchronous, but that's off-topic here.

The problem here is that this mental model doesn't really fit what the browser does. In particular, the browser does nothing until a data channel is first opened, plus the browser can call onclose or onerror on the data channel before onopen. To accomodate for that, the current code is a bit hacky in the sense that it reports the connection as immediately open.

This PR does it a bit more differently and more properly: we immediately open a data channel at the same time as we create the RTCPeerConnection in order for the browser to start connecting, even though smoldot didn't request a channel yet. We store this first data channel on the side, and when smoldot asks for a data channel we provide this one.
If a data channel closes while it is still opening (the handshake one or another), we just kill the entire connection.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[tomaka]

I have a headache from the complexity brought by the fact that there are dozens and dozens of asynchronous functions and that things can be called in many different orders. But I think that such is the nature JS and there's no great way to make the code more easy.

[github-actions]

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