feat: OCS Calendar Import

[SebastianKrupinski]

Summary

Extracted from #49995

This adds the ability to export and import calendars via the OCS

OCS Import

Endpoint: /ocs/v2.php/calendar/import
Request: POST

{
	"transaction": "random string",
	"target": "personal",
	"options": {
		"format": "xcal", (optional "ical, jcal, xcal", defaults to "ical")
		"validation": 0, (0 - no validate, 1 - validate and skip, 2 - validate and error)
		"errors": 0, (0 - continue, 1 - fail)
		"supersede": true,
	},
	"user": "user1", (optional admin permissions required)
	"data": "<?xml version=\"1.0\" encoding=\"UTF-8\"?><calendar><event><title>Meeting</title><date>2025-02-18</date></event></calendar>"
}

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

The code comments in the controller are a bit useless, you might want to remove them to avoid confusion if the code is refactored later.

Stale

[ChristophWurst]

same as #61277 (comment)

[ChristophWurst]

new public API. this is going to be messy in the backport.