Added Enum support

[samson0v]

Added Enum support for Modbus connector.

Configuration example:

{
  "tag": "operational_mode",
  "type": "16int",
  "address": 1,
  "objectsCount": 1,
  "functionCode": 3,
  "variants": {
    "1": "Normal Mode",
    "2": "Service Mode",
    "3": "Calibration Mode"
  }
}

Result on ThingsBoard platform:

Configuration example for coil:

{
  "tag": "relay",
  "type": "bits",
  "address": 1,
  "objectsCount": 1,
  "functionCode": 1,
  "bitTargetType": "int",
  "variants": {
    "0": "OFF",
    "1": "ON"
  }
}

Result on ThingsBoard platform:

Also, the feature works as expected with uplink RPC to Device. For example:

"rpc": [
  {
    "tag": "getOperationalMode",
    "type": "16int",
    "address": 1,
    "objectsCount": 1,
    "functionCode": 3,
    "variants": {
      "1": "Normal Mode",
      "2": "Service Mode",
      "3": "Calibration Mode"
    }
  }
]

Result on ThingsBoard: