Envelope codec format

Nobody has claimed this yet.

Libp2p's record "envelop" allows one to specify a "codec". This is a length-prefixed byte array to allow for either "codec" or "paths" (`/foo/bar`). This isn't ambiguous because `/`, UTF-8 encoded, is the uvarint-encoded multicodec for "path".

Unfortunately, this only works if the leading multicodec is encoded as a uvarint. go-libp2p records use 0x0301, big-endian encoded. When parsed as a uvarint-encoded number, this will be interpreted as CIDv3 (not yet defined) followed by a single 0x1 byte.

The libp2p record codec should have been encoded as `[0x01|(0x1<<7), 0x01] = [0x81, 0x03]` (Little Endian)

Options:

1. Require that the "codec" be a big-endian encoded multicodec (removes the possibility for paths, protocol names, etc).
2. Define it as arbitrary bytes (would make "ipld records" difficult).
3. Fix the encoding with an upgrade period.

Start by reviewing the libp2p record envelope and codec encoding path, then compare the stated big-endian and uvarint interpretations. The issue presents three incompatible options but does not select a resolution; done would require an agreed encoding decision and an upgrade or compatibility plan.

Written by the indexing model from the issue text.