correctly (don't) interpolate non-text values in custom responses

[erohmensing]

Proposed changes:

• revert this change, which turned all template values into strings in interpolation in order to avoid int and None values from breaking templates
• fix if custom utter message contains no value or integer value then rasa fails returning custom utter message #5453 by bypassing interpolation for int, None, and bool values
• avoid turning ints, Nones, and bools into strings, which can negatively affect the type of the value sent to the connector

Note: when 1.10.x is merged to master, we need to revert the change in the above PR, as it won't show in the diff

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[chkoss]

Nice solution, thanks for adding the doc strings, too! 🚀

[chkoss]

About the failing type check: maybe we have to add # pytype: disable=pyi-error to the line 8 in rasa.core.nlg.template, where we do from rasa.core.nlg.generator import NaturalLanguageGenerator.

The issue is probably that in rasa.core.nlg.generator there is a function _create_from_endpoint_config which does from rasa.core.nlg import TemplatedNaturalLanguageGenerator, so the type checker thinks there is a cyclical import. In fact, that isn't really a problem (I think), so we can disable it.