feat: Remove unnecessary gogoproto.jsontag in the did.proto

[youngjoon-lee]

Closes #176

We decided to not make our JSON response as the W3C JSON-LD format: #176 (comment).
But, I just refined the did.proto by removing some unnecessary tags.

[gyuguen]

👍
Thank you. As long as the CI passes, it can be merged!

[youngjoon-lee]

@gyuguen
4ead621 I fixed tests by using jsonpb.Unmarshal to read JSON files in the CLI. The json.Unmarshal expected snake_case because I removed (gogoproto.jsontag). Also, it doesn't handle custom types.

Let's think that the rule of thumb is using jsonpb.Unmarshal to unmarshal proto.Messages, regardless of the field names and custome type definitions. No exception anymore. That's also what Google recommends (like the art of abstraction).
We've went through a long journey, but I think this is the end of the story.

[gyuguen]

I fixed tests by using jsonpb.Unmarshal to read JSON files in the CLI. The json.Unmarshal expected snake_case because I removed (gogoproto.jsontag). Also, it doesn't handle custom types.

I'm checked.
Test passed well, so we can merge it. :)

[gyuguen]

👍