fix: Make field index a string for better id/index support

Signed-off-by: Elizabeth Danzberger <lizzy7128@tutanota.de>

lib/public/Files/Template/Field.php

@@ -10,7 +10,7 @@
 use OCP\Files\Template\InvalidFieldTypeException;
 
 class Field implements \JsonSerializable {
-	private int $index;
+	private string $index;
 	private string $content;
 	private FieldType $type;
 	private ?int $id;