Abstract over DIDs with an async verify interface

[cdata]

DIDs may not always resolve directly to public key bytes. This being the case, the current strategy to resolve DIDs to key material via DidParser is probably not adaptable enough to suit our needs over the long term.

An imminent refactor of ts-ucan will abstract over DIDs with an async verification method: https://github.com/ucan-wg/ts-ucan/blob/0e2d5502576eeb1e524b866e1539a2b35e4aa0c9/packages/core/src/plugins.ts#L9-L12

We should probably follow suit with a similar strategy, for the sake of coherence and long-term flexibility in resolving DIDs.

[cdata]

Note that this has direct relevance to #3