Make webrtc-specific multistream-select negotiation spec compliant

[haikoschol]

This PR brings the WebRTC-specific multistream-select implementation in line with the spec. Before this change, the header, the desired protocol and all fallback protocols were sent in a single multistream-select message. The spec mandates that only the desired protocol is to be sent instead. If the peer responds with a na, indicating it does not support this protocol, another multistream-select message should be sent, containing the first fallback protocol. This is to be repeated until the peer accepts a protocol or the dialer runs out of fallbacks.

This fix is required for interop with smoldot. Without this fix, the following happens:

• The dialer proposes a protocol with fallbacks in a single message (e.g. /multistream/1.0.0\n/c7fd75141ca230c62e99af66d68f9d1f84b0821bcc1e17d2be92aaf5602d1c0e/block-announces/1\n/dot/block-announces/1\n).
• Smoldot reads from its input buffer until it sees /c7fd75141ca230c62e99af66d68f9d1f84b0821bcc1e17d2be92aaf5602d1c0e/block-announces/1\n and sends back a confirmation.
• The fallback protocol remains in the input buffer and is misinterpreted by Smoldot as the block announce handshake. Smoldot does not verify the handshake on inbound substreams.
• The dialer sends the actual block announce handshake (70 byte payload).
• Smoldot misinterprets this as the first block announce message and fails to decode it and logs this error to the browser console: DecodeBlockAnnounceError(Verify).

This PR builds on ChainSafe/litep2p#6 to incorporate the changes to WebRTC protobuf flag handling.

closes #75

[haikoschol]

I meant to open this in our fork first and get a review from @timwu20. Will convert this to draft because it builds on work that should reviewed first in a PR that hasn't been opened in this repo yet.

[haikoschol]

After syncing with @timwu20 we decided that opening a PR for these changes based off master easier for everyone.