Adopt did:pkh #10

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