datagrams: Add initial datagram draft

[MarcoPolo]

Initial text for specifying how datagrams will work on libp2p.

cc @raulk @sukunrt @jxs

[marten-seemann]

Have you considered tying a datagram flow to a stream, similar to what HTTP DATAGRAMS (RFC 9297) does?

This would have the following benefits:

1. Shorter datagram header (especially important given MTU restrictions).
2. Both peers get the opportunity to cancel a datagram flow (by closing / resetting the associated stream).
3. A reliable control channel, e.g. for application protocols that need to send an occasional control message next to a datagram flow.

[MarcoPolo]

Thanks @marten-seemann! That's a nice idea. We can include the versioning information in the stream as well so we don't pay that overhead on every datagram. The datagram itself then only has a single varint of overhead.