[proposal] add did:pkh support

[mirceanis]

Requirements

• inline resolver; we can use the resolver from ceramic: pkh-did-resolver

import { getResolver as getPkhResolver } from 'pkh-did-resolver'
// ...
new DIDResolverPlugin({
  // ...
  ...getPkhResolver(),
})

• @veramo/did-provider-pkh - at least for ethereum
  This requires an implementation of AbstractIdentifierProvider
  Since did:pkh doesn't support updates, this provider would be very similar to @veramo/did-provider-key, in the sense that most methods would be stubs, and that DIDs are created from a key.

The first implementation can support only the ethereum based DIDs, did:pkh:eip155:....
The parameters for creating a DID linked to a particular network should be inspired by @veramo/did-provider-ethr (the same file has a lot of other code that should prove useful).

testing

The PR closing this issue should contain a test where a did:pkh is created and used as an issuer in a call to createVerifiableCredential(), then that credential should be verified successfully using verifyCredential().
It is acceptable to provide only integration tests for this (meaning tests added to the top level __tests__ folder)

constraints

• Environments: All plugin packages published in this repository are meant to work in nodejs, web(react) and mobile(react-native) environments. This will apply to this new package too.
• No new dependencies: Packages in this repository are often installed together so dependencies must be reused between them when possible. No new dependencies should be introduced if this can be avoided.