Calendar invites: Administrator shall choose if server's or user's email address is used

Feature Request I refer to #2345 and #2778. If a Nextcloud instance serves for users from different domains (which is the case at least for me), it's annoying that calendar invites are sent with the server's email address instead of the user's once. For me it would be fine if the server's address is taken if there is no user email configured. To avoid problems with mailservers that might not accept sending emails for more than one domain, this behavior this could be made optional. (In my case all domains are hosted and handled by one email server and one nextcloud server - I would not like to set up a nextcloud instance per domain). (If someone is so nice and point me to the place where I can change it in the code myself, would be fine too) Implementation Delegate sending emails to apps. The main target is the Mail app but others may offer this integration too. Work packages • Add a mail sending provider mechanism • Implement the mail sending p…