[MM-58308] Add number of attachments

[JulienTant]

Summary

Add a footer to the notification letting the user know if file attachments were sent:

Ticket Link

https://mattermost.atlassian.net/browse/MM-58308

[lieut-data]

This looks great, @JulienTant! Could I trouble you to rebase on the (literally just pushed) https://github.com/mattermost/mattermost-plugin-msteams/pull/667 and add unit test coverage for these cases using the new framework? Apologies for moving the code slightly, but happy to help resolve any conflicts :p

[lieut-data]

(Updated link above to https://github.com/mattermost/mattermost-plugin-msteams/pull/667)

[JulienTant]

I was just adding tests :P Will rebase.

[JulienTant]

Rebased, still need to add tests

[JulienTant]

@lieut-data added tests using the permutator :P Please do not refactor your test on your branch 🤣

[JulienTant]

@lieut-data updated this to exclude codeblocks and reply blocks from attachment count.