[Bug] JWT Verifiable Presentations should utilize the iss property

[decentralgabe]

Describe the bug
VPs encoded as JWTs as per the vc-jwt spec should utilize the iss property to represent the holder. The implementation currently uses the sub property, which is incorrect.

To Reproduce

Sample JWT VP generated by the impl:

eyJhbGciOiJFZERTQSIsImtpZCI6InRlc3Qta2lkIiwidHlwIjoiSldUIn0.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.BNzz3AfY5WcfNyZ7ex9KeI1CEJoiO9Tww7Oj12aGKGXWYj7RsNtg0oQwMtUw5L61PqfCZXbzFSM4Sn7MtT10BA

which decodes to

{
  "jti": "5ea1a8ac-a468-453b-aa55-6a0926af5c5b",
  "nonce": "a68442e5-1a88-4090-9b17-7bf54eb93b6f",
  "sub": "did:example:ebfeb1f712ebc6f1c276e12ec21",
  "vp": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1",
      "https://www.w3.org/2018/credentials/examples/v1"
    ],
    "id": "5ea1a8ac-a468-453b-aa55-6a0926af5c5b",
    "holder": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "type": [
      "VerifiablePresentation",
      "HappyPresentation"
    ],
    "verifiableCredential": [
      {
        "@context": [
          "https://www.w3.org/2018/credentials/v1",
          "https://www.w3.org/2018/credentials/examples/v1"
        ],
        "id": "5dd14b9a-c9c2-43c6-8e91-0dcac274d4c6",
        "type": [
          "VerifiableCredential",
          "HappyCredential"
        ],
        "issuer": "https://example.com/issuers/565049",
        "issuanceDate": "2010-01-01T19:23:24Z",
        "credentialSubject": {
          "happiness": {
            "howHappy": "really happy"
          },
          "id": "did:example:ebfeb1f712ebc6f1c276e12ec21"
        }
      }
    ]
  }
}