Adopt did:pkh

https://github.com/w3c-ccg/did-pkh/blob/main/did-pkh-method-draft.md

Instead of, or in addition to `did:key`.

Differences with `did:key`:
- Don't need to sign a message whenever we want to get the DID of an account
- Instead needs work on the backend to lookup the public key so we can verify a signature
- Scoped to the network and chain ID