feat(ocp): calendar event builder api

[st3iny]

• Part of: Schedule meetings inside Talk calendar#6516

Summary

The event builder API is done and ready to use.

Code example

		$startDate = (new \DateTimeImmutable('now'))
			->setTimezone(new \DateTimeZone('Europe/Berlin'));
		$endDate = $startDate->add(new \DateInterval('PT1H'));
		$builder = $this->calendarManager->createEventBuilder()
			->setStartDate($startDate)
			->setEndDate($endDate)
			->setSummary('An Event')
			->setDescription('With a description')
			->setLocation('Some address') // A URL (of a meeting) would also work
			->setOrganizer('[email protected]')
			->addAttendee('[email protected]')
			->addAttendee('[email protected]', 'User Two');

		// Write the calendar to an event
		/** @var ICreateFromString|ICalendar $calendar */
		$builder->createInCalendar($calendar); // Get calendar from \OCP\Calendar\IManager

		// Or, serialize it to a string to do something else with it
		$ics = $builder->toIcs();

TODO

• Actually implement the availability API (ref Schedule meetings inside Talk calendar#6516 (comment))

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)

[st3iny]

@nickvergessen This should get you started for the meetings feature. Let me know if you have questions.

[st3iny]

I added a example on how to use the ICalendarEventBuilder API to create an event.

[nickvergessen]

Looks good, will implement Talk's side now

[nickvergessen]

Is there a max length that we should check, or does caldav take care of that, same for description?