Add command to generate chain link file

[dadamu]

Feature description

Currently, the sign cmd returns the hex-ecoded public key. However, The public key in the proof object is required as base64-encoded string when creating a chain link. From my side, the sign cmd should return the public key as base64-encoded.

The chain link required file:

{
   "address":{
      "@type":"/desmos.profiles.v1beta1.Bech32Address",
      "value":"cosmos15uc89vnzufu5kuhhsxdkltt38zfx8vcyggzwfm",
      "prefix":"cosmos"
   },
   "proof":{
      "pub_key":{
         "@type":"/cosmos.crypto.secp256k1.PubKey",
         "key":"A58DXR/lXKVkIjLofXgST/OHi+pkOQbVIiOjnTy7Zoqo"   /*<- It's base64-encoded*/
      },
      "signature":"ecc6175e730917fb289d3a9f4e49a5630a44b42d972f481342f540e09def2ec5169780d85c4e060d52cc3ffb3d677745a4d56cd385760735bc6db0f1816713be",
      "plain_text":"cosmos15uc89vnzufu5kuhhsxdkltt38zfx8vcyggzwfm"
   },
   "chain_config":{
      "name":"cosmos"
   }
}

Implementation proposal

Let the public key of signature data in sign cmd be base64-encoded.

[RiccardoM]

@dadamu The sign command returns an hex-encoded signature and public key because it should be used when creating application links. The process should be:

1. The user signs its application username using the sign command.
2. The returned output is posted online.
3. The Band data sources will fetch and validate those data expecting an hex-encoded signature and public key.

Changing this command would break everything.

Also, I don't think it's worth changing it and requiring the user to create the chain-link object by themselves. Instead, what we should do is create a new command that allows the user to create such object easily. However, I think this is not a priority right now since the overall JSON object can be created very easily using Protobuf and the JSON encoding of the Cosmos-provided objects. Indeed, the public key value is simply obtained by creating a cryptotypes.PubKey instance and encoding it with the json package.

[dadamu]

@RiccardoM Okay, I am thinking about the same thing that creating a new command in order to generate object easily.
Currently, to have a chain link, the user should use sign command to generate the signature and get base64-encoded public key, then create the such object manually.
I suppose it should be mentioned in docs before the command is out.