Custom NLG class from module string

[pheel]

Description of Problem:
Rasa supports module strings in configuration files, which makes it easy to develop custom file importers, channels, tracker stores, policies, NLU components, etc. One aspect that escapes this customization pattern is the Natural Language Generator class. This could be useful for many reasons, including specifying one's own NLG request schema.

Overview of the Solution:
I propose to use the existing type kwarg of rasa.utils.endpoints.EndpointConfig to specify a module string. Resolved modules would then be initialized with i) the endpoint config (including url and other kwargs) and ii) the domain. If module does not resolve, fallback to the current built-ins.

[tmbo]

Fixed as part of #4801