Add options for powerful appointment booking forms

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

It would be great to keep the necessary infos for well prepared appointments handy directly in the calendar and linked in the nextcloud. For some the phone number is obligatory, others need to know other things or need some documents to prepare an appointment.

Describe the solution you'd like

Booking form: Here it would be very useful if you could define additional form fields with:

• free text
• predefined lists
• radio buttons
• check boxes
• ability to upload files for selected fields. Files should be saved in the nextcloud in an defined folder which automated generated subfolders (one subfolder per appointment)
• abilty to define fields as optional/obligatory
• ability to define charater limit
• even more powerful would be conditional fields (if checkbox A is set to „bla“ then show/do not show field B…)

Describe alternatives you've considered

• Maybe it would be an alternative to integrate the forms app here instead of coding additional forms here but I cannot imagine how this could work.

• Maybe an other attempt could be a possibility for appointment bookers without nextcloud account to provide a link to add this additional infos later?

Additional context

No response

No files, tests, or entry points are named in the issue. Start by reviewing the existing appointment booking form and how the calendar integrates with Nextcloud storage or the Forms app, then narrow the proposal to a defined subset of field types, validation, uploads, and conditional behavior with explicit acceptance criteria.