[Feature request]: Custom Payload Injection

[jm20122012]

Is your feature request related to a problem? Please describe

I cannot find a way to configure custom payload injection.

Describe the solution you'd like

I would like to be able to inject custom key-value pairs into the message that gets broadcast over MQTT. For example, if a temp sensor only exposes "temperature" and "humidity", I would like to inject the friendly name for the device into this message so that the actual message that gets sent out would like like this:

{ "friendly_name": "myAwesomeSensor", "temperature": 30, "humidity": 35 }

Describe alternatives you've considered

None

Additional context

None