Provide dav setting for user's default calendar

[tcitworld]

And add tests to handle schedule-default-calendar-URL

Part 1 for #19826

[georgehrke]

Just double checked to make sure:
The dav property is a protected one: https://github.com/nextcloud/3rdparty/blob/master/sabre/dav/lib/CalDAV/Schedule/Plugin.php#L129

[georgehrke]

@tcitworld What is your concept for the endpoint for setting this?
Some DAV method or just a Controller endpoint?

[tcitworld]

Since the property was read-only in Sabre I only thought of it at first as a Controller endpoint. We could indeed do that through a DAV method, but is it worth it anyway? Does macOS/iOS Calendar handle this?

[georgehrke]

No, the only two interesting things you can configure via DAV in macOS are default alarms for timed events and allday events:

<A:propertyupdate xmlns:A="DAV:">
<A:set>
<A:prop>
<B:default-alarm-vevent-datetime xmlns:B="urn:ietf:params:xml:ns:caldav">
<![CDATA[BEGIN:VALARM
X-WR-ALARMUID:7254E71A-A1F6-41F5-A006-1B435D9B78E3
UID:7254E71A-A1F6-41F5-A006-1B435D9B78E3
TRIGGER:-PT30M
ATTACH;VALUE=URI:Chord
ACTION:AUDIO
END:VALARM
]]>
</B:default-alarm-vevent-datetime>
</A:prop>
</A:set>
</A:propertyupdate>

<A:propertyupdate xmlns:A="DAV:">
<A:set>
<A:prop>
<B:default-alarm-vevent-date xmlns:B="urn:ietf:params:xml:ns:caldav">
<![CDATA[BEGIN:VALARM
X-WR-ALARMUID:659B19ED-7E2B-427A-9ABA-973206876796
UID:659B19ED-7E2B-427A-9ABA-973206876796
TRIGGER:-P1DT15H
ATTACH;VALUE=URI:Chord
ACTION:AUDIO
END:VALARM
]]>
</B:default-alarm-vevent-date>
</A:prop>
</A:set>
</A:propertyupdate>

Changing the default calendar does not result in any CalDAV requests.

[nickvergessen]

Looks good in general

[skjnldsv]

Conflicts

[tcitworld]

Rebased and php-cs-fixed :)