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/Repository/BlobIndex.php

@@ -61,7 +61,6 @@ final class BlobIndex {
 		fwrite($stream, $this->cipher->encryptString(json_encode($entries, JSON_THROW_ON_ERROR), 'index:' . $packId));
 		rewind($stream);
 		$this->backend->put('index/' . $packId, $stream);
-		fclose($stream);
 		foreach ($entries as $e) {
 			$this->blobs[$e['id']] = [$packId, $e['offset'], $e['length'], $e['raw'], $e['flags']];
 			unset($this->pending[$e['id']]);