`multistream-select` implementation used by WebRTC doesn't conform to the spec

[dmitry-markin]

Currently, we encode and send all protocols at once during multistream-select phase when opening WebRTC stream:

litep2p/src/multistream_select/dialer_select.rs

Lines 273 to 278 in 337538f

	let message = encode_multistream_message(
	std::iter::once(protocol.clone())
	.chain(fallback_names.clone())
	.filter_map(|protocol| Protocol::try_from(protocol.as_ref()).ok())
	.map(|protocol| Message::Protocol(protocol)),
	)?

This is against libp2p multistream-select spec, which states that protocols should be negotiated one-by-one.

Also, the received protocols are also expected to be sent as a list:

litep2p/src/multistream_select/dialer_select.rs

Lines 294 to 295 in 337538f

	let Message::Protocols(protocols) =
	Message::decode(payload.into()).map_err(|_| Error::InvalidData)?

This also doesn't conform to the spec.

Nevertheless, the above works with smoldot. This issue is about fixing the implementation, and, may be, filing a bug to smoldot if it doesn't conform to the spec either.

Originally discussed in #51 (comment).

[haikoschol]

I should have looked for existing issues before opening #464, but there I describe my findings and have opened a PR with proposed fixes.

Something must have changed in the meantime though, because in my testing of interoperability between a "minimal smoldot" dialer and a minimal litep2p listener, multistream-select negotiation for outbound substreams from litep2p does not work (i.e. the second part in this issues description) without my fixes.