Add possibility to split the domain into separate files

[degiz]

Closes #5984

Proposed changes:

• domain file now doesn't have to be called domain.yaml

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)

[federicotdn]

Works nicely! Some observations:

• We should update default_arguments.add_domain_param as well. Maybe mention there that the value can be a single YAML file, or a directory containing YAML files.
• I tried placing three YAML files inside domains/ and then trained my model. However I got this warning below:

FutureWarning: No tracker session configuration was found in the loaded domain. Domains without a session config will automatically receive a session expiration time of 60 minutes in Rasa version 2.0 if not configured otherwise.
  docs=DOCS_URL_DOMAINS + "#session-configuration",

I think this is happening because we're calling Domain.from_dict for each file, so if one of them doesn't have the session_config key, the warning will be shown.

[degiz]

FutureWarning: No tracker session configuration was found in the loaded domain. Domains without a session config will automatically receive a session expiration time of 60 minutes in Rasa version 2.0 if not configured otherwise.
docs=DOCS_URL_DOMAINS + "#session-configuration",

I removed the warning and set the default session expiration time to 60 minutes as of now.

[degiz]

@federicotdn update: used DEFAULT_SESSION_EXPIRATION_TIME_IN_MINUTES everywhere and still keep it as 0 now. This question needs more clarification and I don't want it to block the PR 😬

[federicotdn]

👍
Let me know if I should review again

[federicotdn]

👍 👍