feat(ocp): add calendar api to retrieve availability of attendees

[st3iny]

• Part of: Schedule meetings inside Talk calendar#6516

Summary

Add a public API to check if a list of attendees are available in a given time slot. Note that for now, the email addresses of the given attendees have to belong to users on the same instance.

Usage example

$start = new DateTimeImmutable('now');
$result = $this->calendarManager->checkAvailability(
	$start,
	$start->add(new \DateInterval('PT2H')),
	$organizer, // IUser(uid=admin, [email protected])
	['[email protected]', '[email protected]']
)

// Result (see \OCP\Calendar\IAvailabilityResult):
// $result[0]->getAttendeeEmail() === '[email protected]'
// $result[0]->isAvailable() === false
// $result[1]->getAttendeeEmail() === '[email protected]'
// $result[1]->isAvailable() === true
// $result[2]->getAttendeeEmail() === '[email protected]'
// $result[2]->isAvailable() === false

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)

[nickvergessen]

Looks like it can do the job, but didn't test

[ChristophWurst]

Looks good otherwise :)

[ChristophWurst]

Looks good!