Define metadata type for Trustless HTTP Gateway Multicodec

[hannahhoward]

What

Define a metadata type for the new multicodec defined here multiformats/multicodec#321, support records publishing for this metadata type

Implementation

My suggestion is for the time being, this metadata should simply be an empty struct until we identify any additional parameters required. The codec isn't really finished until ipfs/specs#402 is merged, and I expect more things to change before it is.

[willscott]

agree it can be empty - the same as the current bitswap behavior.
for that, i don't believe any particular change is needed to support it, though we can provide a nice name in https://github.com/ipni/go-libipni/tree/main/metadata rather than making users use the unknown fallback wrapper.

[hannahhoward]

That'd be great!