Locations: edit connection settings without losing the history

A changed password or host name could only be handled by adding a new
location, which starts an empty repository and, with a retention lock at
the provider, keeps the old one billed. Edit now changes a location in
place (web interface and occ backup:target:edit). The settings are only
saved when they lead to the same repository; a failed connection and a
missing repository give separate errors, and the folder is not created
while checking. Stored passwords are never sent to the browser: an empty
password field keeps the stored value.

lib/Backend/StorageBackend.php

@@ -21,11 +21,12 @@ final class StorageBackend implements IBackend {
 	public function __construct(
 		private IStorage $storage,
 		private string $base = '',
+		bool $createBase = true,
 	) {
 		$this->base = trim($base, '/');
 		self::validate($this->base, true);
 		$this->atomicPut = $storage->instanceOfStorage(\OCA\Files_External\Lib\Storage\AmazonS3::class);
-		if ($this->base !== '') {
+		if ($createBase && $this->base !== '') {
 			$this->mkdirs($this->base);
 		}
 	}
@@ -109,6 +110,15 @@ final class StorageBackend implements IBackend {
 		}
 	}
 
+	/** Whether the storage accepts the connection settings (files_external's own check). */
+	public function canConnect(): bool {
+		try {
+			return !method_exists($this->storage, 'test') || (bool)$this->storage->test();
+		} catch (\Throwable) {
+			return false;
+		}
+	}
+
 	public function exists(string $path): bool {
 		return $this->storage->file_exists($this->abs($path));
 	}