Revert "[camera] Remove deprecated Optional type (flutter#6870)"

Removing Optional from CameraValue.copyWith caused a regression handling device orientation changes while the preview was paused.

This reverts commit 3d8b73b.

# Conflicts:
#	packages/camera/camera/CHANGELOG.md
#	packages/camera/camera/lib/src/camera_controller.dart
#	packages/camera/camera/pubspec.yaml
#	packages/camera/camera_android/CHANGELOG.md
#	packages/camera/camera_android/pubspec.yaml
#	packages/camera/camera_avfoundation/CHANGELOG.md
#	packages/camera/camera_avfoundation/pubspec.yaml

Author: miracoli

packages/camera/camera/test/camera_preview_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:quiver/core.dart';
 
 class FakeController extends ValueNotifier<CameraValue>
     implements CameraController {

packages/camera/camera/test/camera_test.dart

@@ -13,6 +13,7 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:mockito/mockito.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
+import 'package:quiver/core.dart';
 
 List<CameraDescription> get mockAvailableCameras => <CameraDescription>[
       const CameraDescription(

packages/camera/camera_android/example/lib/camera_controller.dart

@@ -9,6 +9,7 @@ import 'package:camera_platform_interface/camera_platform_interface.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:quiver/core.dart';
 
 /// The state of a [CameraController].
 class CameraValue {

packages/camera/camera_avfoundation/example/lib/camera_controller.dart

@@ -9,6 +9,7 @@ import 'package:camera_platform_interface/camera_platform_interface.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:quiver/core.dart';
 
 /// The state of a [CameraController].
 class CameraValue {