[feature-request] Create a template model class

[DonnC]

Create an engine template model class and a message type model class to make it easy to work with.

For example

class EngineTemplate:
    typ: str
    message: BaseTemplateMessage
    params: Dict
    routes: Dict
    # ...

Each template message type can inherit BaseTemplateMessage

class BtnMessage(BaseTemplateMessage):
    body: str
    buttons: List[str]
    title: Union[str, None]
    # ...

This will greatly simplify working with templates especially with Dynamic templates in hooks