feat: setIndexConfigurationFromJSON() API. Allow users to pass JSON string (#10029)

* feat: `setIndexConfigurationFromJSON()` API

* analyse issue

Author: russellwheatley

packages/cloud_firestore/cloud_firestore/example/integration_test/instance_e2e.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -229,6 +230,30 @@ void runInstanceTests() {
           fieldOverrides: [fieldOverride1, fieldOverride2],
         );
       });
+
+      test('setIndexConfigurationFromJSON()', () async {
+        final json = jsonEncode({
+          'indexes': [
+            {
+              'collectionGroup': 'posts',
+              'queryScope': 'COLLECTION',
+              'fields': [
+                {'fieldPath': 'author', 'arrayConfig': 'CONTAINS'},
+                {'fieldPath': 'timestamp', 'order': 'DESCENDING'}
+              ]
+            }
+          ],
+          'fieldOverrides': [
+            {
+              'collectionGroup': 'posts',
+              'fieldPath': 'myBigMapField',
+              'indexes': []
+            }
+          ]
+        });
+
+        await firestore.setIndexConfigurationFromJSON(json);
+      });
     },
   );
 }

packages/cloud_firestore/cloud_firestore/lib/src/firestore.dart

@@ -323,6 +323,20 @@ class FirebaseFirestore extends FirebasePluginPlatform {
     return _delegate.setIndexConfiguration(json);
   }
 
+  /// Configures indexing for local query execution. Any previous index configuration is overridden.
+  ///
+  /// The index entries themselves are created asynchronously. You can continue to use queries that
+  /// require indexing even if the indices are not yet available. Query execution will automatically
+  /// start using the index once the index entries have been written.
+  /// See Firebase documentation to learn how to configure your index configuration JSON file:
+  /// https://firebase.google.com/docs/reference/firestore/indexes
+  ///
+  /// This API is in preview mode and is subject to change.
+  @experimental
+  Future<void> setIndexConfigurationFromJSON(String json) async {
+    return _delegate.setIndexConfiguration(json);
+  }
+
   @override
   // ignore: avoid_equals_and_hash_code_on_mutable_classes
   bool operator ==(Object other) =>