support erc1056 for did document

[zwalo]

I would like to develop a ssi service using your package.

The id document of the ssi service I am developing will support "blockchainAccountId" rather than public key in the "verificationMethod" field based on the example of the link I attached below to support compatibility with the ac-1056 based DIDRegistry convention.

https://github.com/decentralized-identity/ethr-did-resolver/blob/master/doc/did-method-spec.md
https://identity.foundation/EcdsaSecp256k1RecoverySignature2020/

Therefore, I modified it to support the verification method generation function of Ecdsaecp256k1 RecoveryMethod2020 type to support that part of your package.

The code below is a logic that created a document using the code I modified :)

        newDid := "did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a"
	didInfo, err := did.ParseDID(newDid)
	if err != nil {
		return nil, err
	}

	// Empty did document:
	doc := &did.Document{
		Context: []interface{}{did.DIDContextV1URI(), did.SECP256RecoveryURI()},
		ID:      *didInfo,
	}

	// Add an assertionMethod
	keyID, _ := did.ParseDIDURL(fmt.Sprintf("%s#key-1", newDid))

	keyID.DID.MethodID = common.Big1
	didInfo.MethodID = common.Big1

	verificationMethod, err := did.NewVerificationMethod(*keyID, ssi.ECDSASECP256K1RecoveryMethod2020, *didInfo, nil)
	if err != nil {
		return nil, err
	}

	// This adds the method to the VerificationMethod list and stores a reference to the assertion list
	doc.AddAssertionMethod(verificationMethod)

	didJson, _ := json.MarshalIndent(doc, "", "  ")
	fmt.Println(string(didJson))

result :

{
  "@context": [
    "https://www.w3.org/ns/did/v1",
    "https://w3id.org/security/suites/secp256k1recovery-2020/v2"
  ],
  "assertionMethod": [
    "did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a#key-1"
  ],
  "id": "did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a",
  "verificationMethod": [
    {
      "blockchainAccountId": "eip155:1:0xb9c5714089478a327f09197987f16f9e5d936e8a",
      "controller": "did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a",
      "id": "did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a#key-1",
      "type": "EcdsaSecp256k1RecoveryMethod2020"
    }
  ]
}

[woutslakhorst]

Thank you for your contribution to this lib. Unfortunately I see 2 problems with your PR:

1. The dependency on the ethereum library (not needed IMHO if it's only used for a hashing alg)
2. The addition of non DID-core fields (blockchainAccountId)

What could be done is to add the (Document d) UnmarshalVerificationMethod(target any) method, that allows the use of any custom VerificationMethod. This would also require storing a Raw version when parsing a VM. Would that be sutiable for your use case?