Allow URIs in addition to fingerprints?

[milesmcc]

@wiktor-k wrote as a review to proposed changes in v04:

I'll mention one interesting thing that may be relevant (or not). Another I-D - security.txt uses URIs to refer to PGP keys. That gives them ability to cover in one consistent way a) OPENPGPKEY DNS based keys, b) V4 fingerprints and even c) HTTPS links to keys.

I responded:

Hmm, that's interesting. Might be something we include in v05? @micahflee would allowing URIs seriously complicate things in the implementation? If not, we might consider including it. It'd resolve a lot of the ambiguity around fingerprints, which kinds to allow, and give more flexibility in key sourcing.

Maybe it's something we should consider adding.