Add a method to get estimated export size in IMigrator

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

lib/public/UserMigration/IMigrator.php

@@ -87,6 +87,14 @@ interface IMigrator {
 	 */
 	public function getVersion(): int;
 
+	/**
+	 * Returns an estimate of the exported data size in KiB.
+	 * Should be fast, favor performance over accuracy.
+	 *
+	 * @since 24.0.0
+	 */
+	public function getExportEstimatedSize(): int;
+
 	/**
 	 * Checks whether it is able to import a version of the export format for this migrator
 	 * Use $importSource->getMigratorVersion($this->getId()) to get the version from the archive