CalDAV: handle email aliases for users

[miaulalala]

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Is your feature request related to a problem? Please describe.
Currently, email aliases are not considered when processing a scheduling event in CalDAV. A user anna@nextcloud.com might be invited to an event, but the reply could come from anna.larch@nextcloud.com, which is functionally the same email address; nonetheless the user will be considered a party crasher and the original ATTTENDEE will never have its PARTSTAT updated.

This is especially problematic for iMIP messages, as the email addresses are the primary means of identifying the replying user.

Describe the solution you'd like
CalDAV should have a way to access email aliases for users. A way to do this could be to create a specific service that accesses and compares email adresses to whatever a VEVENT has stored in the ATTENDEE / ORGANIZER property.

Describe alternatives you've considered

Additional context
Follow up to #40876

[tcitworld]

Is this the same as nextcloud/calendar#4918 ? Might be fixed by #31029

[miaulalala]

ah nice, didn't see that. I will check if that works. Thanks Thomas!