Use pydantic in trigger service (#89)

* Use dict-like parameter mapping based on ChainMap rather than passing around an inner
  function
* Declare pydantic models for all trigger service methods. These automatically handle
  validation and type conversion from a dictionary-type mapping, e.g. using the
  ChainMap-based approach above
* Use pydantic.validate_arguments decorator to enable automatic validation and conversion
  to the pydantic model

This removes a lot of boiler plate type conversions (e.g. removes 11 x 
bool(parameters("automatic"))) or parameter validation/error handling, and allows us to
clearly define up front exactly what message parameters a given trigger requires.

Author: rjgildea