Add merge infos to MultiProjectImporter

[neo-alex]

Proposed changes:

• Add async caching to MultiProjectImporter (use cache to avoid re-loading & re-logging domain/stories/nlu/config)
• Add logs for merging data (add actions/entities/forms/intents/slots merge infos in logs): this gives better insights to the user to make sure that the merges are intentional (cf. example below)

Status (please check what you already did):

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

[sara-tagger]

Thanks for submitting a pull request 🚀 The assigned reviewer will take a look at it as soon as possible✨

[CLAassistant]

All committers have signed the CLA.

[wochinge]

@neo-alex How is it going? Can I help with the required changes?

[neo-alex]

@wochinge Fine thanks, sorry for the delay - I will try to do required changes today and will let you know in case I need some help :)

[neo-alex]

Alright, I tried to answer all your comments above with associated micro-commits. Let me know what you think when you get a chance to review again @wochinge Cheers

[tmbo]

@wochinge do you think this should be wrapped up and go into 2.0?

[wochinge]

@tmbo I don't think it's crucial for 2.0 and everybody has enough on their plate already. Will add it to my todo list anyway to make sure this gets wrapped up at some point.

[stale[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.