Add key function to filterx

[jszigetvari]

When we have a parsed SDATA structure in JSON, sometimes we need to work with the actual names of the keys.
This is currently very difficult, so we would need to add a helper function for that.

            "sdata_values": {
              "nm_MobilityAnalyticsAppData@11912": {
                "MobilityPID": "01BF47AA41BDE17800505995159C002",
                "DeviceName": "Wemmet-5547",
                "UserName": "tim.smith",
                "PhoneNumber": "2065551234",
                "CurrentCarrier": "AT&T",
                "AppName": "CustomApp1",
                "AppType": "User",
                "Bytes": "1116842",
                "DateHour": "2017-02-26T19:00:00.000Z"
              }
            }

In this case I would like to get the nm_MobilityAnalyticsAppData@11912 string, without knowing its content beforehand. Something like this would be sufficient, although the final usage may be different:

keys(sdata_values)[0];

[jszigetvari]

We must not forget to update the relevant application configs once this function becomes available.