feat: hide caldav server settings if no app uses the caldav backend

[st3iny]

• Resolves: none

Summary

This PR introduces a new top level key in info.xml called backends which can be used by apps to express their intent to use a specific server-side backend. Currently, the only backend to be required is caldav.

The caldav server admin settings will be hidden if no app is requiring the caldav backend.

Exemplary info.xml

<?xml version="1.0"?>
<info xmlns:xsi= "http://www.w3.org/2001/XMLSchema-instance"
	  xsi:noNamespaceSchemaLocation="https://apps.nextcloud.com/schema/apps/info.xsd">
	<id>calendar</id>
	<name>Calendar</name>
	<summary>A Calendar app for Nextcloud</summary>
	<version>4.8.0-alpha.1</version>
	<namespace>Calendar</namespace>
	<!-- [...] -->
	<backends>
		<caldav />
	</backends>
</info>

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)

[ChristophWurst]

This PR introduces a new top level key in info.xml called backends which can be used by apps to express their intent to use a specific server-side backend. Currently, the only backend to be required is caldav.

If we are changing info.xsd we can also move this into the dependencies element. I think that makes the meaning clearer. The current approach is ambiguous as it could mean that the apps that list caldav as backend are providing it.