feat: 🎸 add W3C-DID as supported type for issuer identityProof

[AdamJLemmon]

• add W3C-DID as a supported identityProof type in schema 3.0
• add tests where type: W3C-DID and location: did:ethr:0x...

[Nebulis]

I will have a better look tomorrow but questions that comes into my mind:

• why using W3C in "W3C-DID" ? Is there multiple DID specifications ?
• would be nice to have one e2e tests (I can help to create the test for this)
• we need to create a verifier in https://github.com/Open-Attestation/oa-verify (I think it should come along this)
• what about v2 schema, do we have needs @rjchow ?

[Nebulis]

• Let's stick to W3C-DID (it's more explicit and specific on what's it's doing)
• Let's only extend v3 schema

[Nebulis]

That's weird, circle didnt run

[john-dot-oa]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: