Add dispatching delegates to implement plugins switching per Config (such as "target").

[dmikurube]

@muga Can you have a look?

It's to implement plugins switching its behavior per Config. Imagine an input plugin from a kind of weather service, for example:

in:
  type: weather_service
  target: temperature  # Ingests from https://weather.example.com/api/temperature

in:
  type: weather_service
  target: precipitation  # Ingests from https://weather.example.com/api/precipitation

This extended Dispatching-Delegate classes make implementing such plugins easier and simpler.

[dmikurube]

Thanks!