NgsiPayloadAttr model in subcriptions should not validate value type

[djs0109]

Describe the bug
NgsiPayloadAttr is now inheriting the BaseValueAttribute, where the value will be validated against the assigned data type.
However, this causes problems for for example the following notification

  "notification": {
    "timesSent": 2,
    "mqttCustom": {
      "url": "mqtt://134.130.166.184:1883",
      "topic": "custom/mqtt/notification/ngsi",
      "qos": 0,
      "ngsi": {
        "id": "newId",
        "type": "newType",
        "attribute_ngsi": {
          "type": "Number",
          "value": "${attribute1}"
        }
      }
    },
    "attrs": [],
    "attrsFormat": "normalized",
    "onlyChangedAttrs": false,
    "covered": false
  }

"value": "${attribute1}" can not pass through the validation against number data type.

[github-actions]

Branch 350-NgsiPayloadAttr-model-in-subcriptions-should-not-validate-value-type created!