API response consistency

[stakersunion]

Describe the bug
Currently, the response from endpoints returning similar data types are inconsistent.

GET /api/v1/validator/eth1/{eth1address}

{
  "data": [
    {
      "public_key": "string",
      "valid_signature": true,
      "validator_index": 0
    }
  ],
  "status": "string"
}

GET /api/v1/validator/withdrawalCredentials/{withdrawalCredentialsOrEth1address}

{
  "data": [
    {
      "publickey": "string",
      "validatorindex": 0
    }
  ],
  "status": "string"
}

Expected behavior
Consider using common key names (ex. include underscore).