Add backup download route if backend supports it

src/storage/interface.js

@@ -11,6 +11,7 @@ exports = module.exports = {
     getAppBackupDetails: getAppBackupDetails,
 
     getRestoreUrl: getRestoreUrl,
+    getLocalFilePath: getLocalFilePath,
 
     copyObject: copyObject
 };
@@ -52,6 +53,17 @@ function getRestoreUrl(apiConfig, filename, callback) {
     callback(new Error('not implemented'));
 }
 
+function getLocalFilePath(apiConfig, filename, callback) {
+    assert.strictEqual(typeof apiConfig, 'object');
+    assert.strictEqual(typeof filename, 'string');
+    assert.strictEqual(typeof callback, 'function');
+
+    // Result: { filePath: <localFilePath> }
+    // The resulting filePath is a local path to the backup file
+
+    callback(new Error('not implemented'));
+}
+
 function copyObject(apiConfig, from, to, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof from, 'string');