fix: Accept a did_base64 for querying a DID via REST

[youngjoon-lee]

Fixes #173

I tested this manually. We should have integration tests for REST API. I'll open another issue.

The response body below has snake_case, but we want for the document to obey the W3C spec (camelCase). Let's try to fix it as a next PR.

$ echo -n "did:panacea:4Fsrryj58HP3zxRE9FPBcsG4EehAmoSuGXuvvpxbrziF" | base64 | pbcopy
$ curl -X GET http://localhost:1317/panacea/did/v2/dids/ZGlkOnBhbmFjZWE6NEZzcnJ5ajU4SFAzenhSRTlGUEJjc0c0RWVoQW1vU3VHWHV2dnB4YnJ6aUY="

{
  "did_document_with_seq": {
    "document": {
      "contexts": [
        "https://www.w3.org/ns/did/v1"
      ],
      "id": "did:panacea:4Fsrryj58HP3zxRE9FPBcsG4EehAmoSuGXuvvpxbrziF",
      "controller": null,
      "verification_methods": [
        {
          "id": "did:panacea:4Fsrryj58HP3zxRE9FPBcsG4EehAmoSuGXuvvpxbrziF#key1",
          "type": "Secp256k1VerificationKey2018",
          "controller": "did:panacea:4Fsrryj58HP3zxRE9FPBcsG4EehAmoSuGXuvvpxbrziF",
          "public_key_base58": "ecfi6w7BeaWmumQ1s7nHvcMasAWhpPkXNoPZ3eXyERLU"
        }
      ],
      "authentications": [
        "did:panacea:4Fsrryj58HP3zxRE9FPBcsG4EehAmoSuGXuvvpxbrziF#key1"
      ],
      "assertion_methods": [
      ],
      "key_agreements": [
      ],
      "capability_invocations": [
      ],
      "capability_delegations": [
      ],
      "services": [
      ]
    },
    "sequence": "0"
  }
}

[gyuguen]

👍