Make the plugin more generic to support multiple asset based on payload or topic

[chicco785]

if this is of interest to the community, we would like to make a pr out of our custom fork:
develop...zaphiro-technologies:fledge-south-mqtt:extract-asset-from-message-or-topic

happy to adapt the code to your suggestions, basically the idea is to support extraction of the asset

1. from a topic pattern. e.g., the value of + in measurement/+/value)
2. from the payload e.g.:

{
  "asset": "my-asset-id-2",
  "readings": {
    "current": 10.29,
    "voltage": 23.82
  }
}