Phase 0: repository on files_external storages without a mount (spike D)

ExternalStorageFactory builds storages with files_external's own
backends and auth mechanisms from an unsaved StorageConfig (as its
ConfigAdapter does), and StorageBackend adapts any IStorage to IBackend
(writeStream, seekable range reads, temp name + rename except on S3).
IBackend::put() now owns and always closes the stream, because
files_external's S3 writeStream closes it.

Spike D (NC34): S3 (SeaweedFS) and SFTP: 120 MiB backup ~6s, unchanged
run uploads nothing, restore bit-identical, no files_external mount
created, no temp files (max 12 KiB in /tmp). Works with files_external
disabled when its classes are loaded. Backup peak memory 85-89 MiB on
these targets (to investigate).

lib/Backend/IBackend.php

@@ -12,7 +12,11 @@ namespace OCA\NgBackup\Backend;
  * Implementations: a local directory (spike, tests), and adapters over files_external storages.
  */
 interface IBackend {
-	/** Store an object; the stream is read to EOF. Must not leave a partial object behind on failure. */
+	/**
+	 * Store an object; the stream is read to EOF. Takes ownership of the stream: it is always
+	 * closed afterwards (some storages, like files_external's S3, close it themselves).
+	 * Must not leave a partial object behind on failure.
+	 */
 	public function put(string $path, $stream): void;
 
 	/** @return resource readable stream of the whole object */