RPC calls signdata and checksignature

[8lecramm]

Related to #79.

If input is given in plaintext, "plain" should be set to 'true'.
The same applies to the response message of 'checksignature'.

curl http://localhost:40403/json_rpc -d '{"jsonrpc":"2.0","id":"0","method":"signdata","params":{"data":"Hello from mmarcel :)","plain":true}}' -H 'Content-Type: application/json' | jq .

{
  "jsonrpc": "2.0",
  "id": "0",
  "result": {
    "signed_data": "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"
  }
}

curl http://localhost:40403/json_rpc -d '{"jsonrpc":"2.0","id":"0","method":"checksignature","params":{"data":"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","plain":true}}' -H 'Content-Type: application/json' | jq .

{
  "jsonrpc": "2.0",
  "id": "0",
  "result": {
    "signer": "deto1qyrg0hj3uytxm7zalxuu7npxwspafwna76d9q9a7wcvtzxxm2v7wcqgfraaxf",
    "message": "Hello from mmarcel :)"
  }
}

[CaptainDero]

Can we have better clarity on structure field names so that things can have better inference. Like during signature check data will always be hex so better name something which shows hex data type etc.