Add support for multiple subjects in a Verifiable Credential

[provTheodoreNewell]

To improve conformance with the Verifiable Credential data model v1.1, add support for multiple subjects in a Verifiable Credential.

From the following excerpt at https://www.w3.org/TR/vc-data-model/#credential-subject, this entails supporting an array of subjects in the credentialSubject field in addition to an object.

It is possible to express information related to multiple subjects in a verifiable credential. The example below specifies two subjects who are spouses. Note the use of array notation to associate multiple subjects with the credentialSubject property.

EXAMPLE 7: Specifying multiple subjects in a verifiable credential

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "id": "http://example.edu/credentials/3732",
  "type": ["VerifiableCredential", "RelationshipCredential"],
  "issuer": "https://example.com/issuer/123",
  "issuanceDate": "2010-01-01T00:00:00Z",
  "credentialSubject": [{
    "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
    "name": "Jayden Doe",
    "spouse": "did:example:c276e12ec21ebfeb1f712ebc6f1"
  }, {
    "id": "did:example:c276e12ec21ebfeb1f712ebc6f1",
    "name": "Morgan Doe",
    "spouse": "did:example:ebfeb1f712ebc6f1c276e12ec21"
  }]
}

I will propose a pull request with an implementation.