XOAuth 2 support

[vatel]

GMail and other mail services support (and highly recommend to use) XOAuth2 for authentication instead of passwords.
Is it possible to implement it in IMAPClient?

[no-identd]

See also:
https://techcommunity.microsoft.com/t5/exchange-team-blog/announcing-oauth-2-0-support-for-imap-and-smtp-auth-protocols-in/ba-p/1330432

And:

https://techcommunity.microsoft.com/t5/exchange-team-blog/announcing-oauth-support-for-pop-in-exchange-online/ba-p/1406600

Albeit full Microsoft Modern Authentication would probably require a whole additional library at the very least, and it's probably not an exact fit for this library anyway, in some sense. Also makes me wonder whether I should raise a separate issue for Microsoft Graph API support, which would seem more fitting to this library.