Update crates to support DID methods other than key

[cdata]

Currently we have built everything around the most essential - and most limited - DID method: key. Key is great because it is very simple and easy to get going with. Unfortunately, it has no facility for rotations, which is something we will need in time. And, there may be other reasons to reach for non-key methods as well. Share 'em if you got 'em!

This issue depends on ucan-wg/rs-ucan#3 & ucan-wg/rs-ucan#16