Authentication for calendar integration

[KatrinIhler]

Already discussed peripherally in #2938.

To use the iCal integration that puts events from my calendar into the timeline, I have to create a public subscription link. This causes some concerns in regards to privacy, as the information in my calendar is only protected by the obscurity of the link.

It would be great if I could configure the private link generated by Nextcloud and my credentials instead.

I'm not sure if this means extending the global calendar integration or whether this makes more sense for the calDAV integration of each project, which does support authentication, but only supports syncing tasks, not calendar entries, right?

[github-actions]

Thank you very much for opening up this issue! I am currently a bit overwhelmed by the many requests that arrive each week, so please forgive me, if I fail to respond personally. I am still very likely to at least skim read your request and I'll probably try to fix all (real) bugs if possible and I will likely review every single PR being made (please, give me a heads up if you intent to do so) and I will try to work on popular requests (please upvote via thumbs up on the original issue) whenever possible, but trying to respond to every single issue over the last years has been kind of draining and I need to adjust my approach for this project to remain fun for me and to make any progress with actually coding new stuff. Thanks for your understanding!