Support multiple scopes in DAV search

[artonge]

Instead of making the search on the targeted folders, the current approach does it on the user's root folder, with some extra conditions applied on the path.

This allows the following scope in WebDAV SEARCH requests:

<d:from>
	<d:scope>
		<d:href>/files/user1/a</d:href>
		<d:depth>infinity</d:depth>
	</d:scope>
	<d:scope>
		<d:href>/files/user1/b</d:href>
		<d:depth>infinity</d:depth>
	</d:scope>
</d:from>

This can be tested with the following PR in Photos: nextcloud/photos#2426
With that PR, the SQL request will look like that:

SELECT
	[...]
FROM
	[...]
WHERE
	[...]
	AND (
		path COLLATE utf8mb4_general_ci LIKE "files/a/%"    -- <-- ADDED CONDITION
		OR path COLLATE utf8mb4_general_ci LIKE "files/b/%" -- <-- ADDED CONDITION
	)
	AND (
		storage = 1
		AND (
			path = "files"
			OR path LIKE "files/%"
		)
	)
ORDER BY
	[...]
LIMIT
	[...]

• Contributes to Add Support to additional folders by changing reworking "Choose another folder" to "Add folder" photos#2343

[icewind1991]

If there is only a single folder, doing the search on the folder is more efficient as it can filter down the storages it searches in

[emoral435]

This LGTM. In terms of optimization, I am generally unknowledgeable here, but the code logic makes sense :)

[artonge]

/backport to stable29