Follow Up: write REQUEST type iMIP messages to calendars automatically

Is your feature request related to a problem? Please describe.

This is a follow up to https://github.com/nextcloud/mail/issues/6802

The difficulty is which calendar the invitation should be written to.

Options:

1. User can define this invitation calendar as a default option somewhere
2. always write to the personal calendar and let the user handle the moving of calendar objects in the Calendar app

Describe the solution you'd like

To Do

• Add an if clause in the IMIpService::process function that checks for 'REQUEST'. This clause should then

  • either check for the existence of the personal calendar or
  • check for the preset calendar from oc_prefrences

• process the REQUEST by calling the ICreateFromString::createFromString method and writing to that specific calendar.

• Mark the message as processed, or in case the storing didn't work, mark it as erroneous.

• In the frontend, if we go with option 1), add a settings field somewhere and store it in oc_preferences.

• Display the processed / erroneous state

Describe alternatives you've considered

Not processing the messages and leaving it up to the user if they want to see them in their calendar. We already provide an import option.

Additional context

No response

Start with IMIpService::process and the ICreateFromString::createFromString entry point described in the issue, then review how calendar selection and message processing states are represented. The open design choice is whether to use the personal calendar or a preference-selected calendar. Done means REQUEST messages are stored in the chosen calendar, marked processed or erroneous, and the option 1 frontend setting and state display are covered if selected.