add margin to vertical stepper (#86067)

* add marging to vertical stepper

* Remove tralling spaces

* Update docstring

packages/flutter/lib/src/material/stepper.dart

@@ -186,6 +186,7 @@ class Stepper extends StatefulWidget {
     this.onStepCancel,
     this.controlsBuilder,
     this.elevation,
+    this.margin,
   }) : assert(steps != null),
        assert(type != null),
        assert(currentStep != null),
@@ -284,6 +285,9 @@ class Stepper extends StatefulWidget {
   /// The elevation of this stepper's [Material] when [type] is [StepperType.horizontal].
   final double? elevation;
 
+  /// custom margin on vertical stepper.
+  final EdgeInsetsGeometry? margin;
+
   @override
   State<Stepper> createState() => _StepperState();
 }
@@ -611,7 +615,7 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
         AnimatedCrossFade(
           firstChild: Container(height: 0.0),
           secondChild: Container(
-            margin: const EdgeInsetsDirectional.only(
+            margin: widget.margin ?? const EdgeInsetsDirectional.only(
               start: 60.0,
               end: 24.0,
               bottom: 24.0,

packages/flutter/test/material/stepper_test.dart

@@ -952,5 +952,43 @@ void main() {
      );
 
      expect(material.elevation, 2.0);
+   });
+
+       testWidgets('Stepper custom margin', (WidgetTester tester) async {
+
+      const EdgeInsetsGeometry margin = EdgeInsetsDirectional.only(
+        bottom: 20,
+        top: 20,
+      );
+
+     await tester.pumpWidget(
+       MaterialApp(
+         home: Material(
+           child: SizedBox(
+             width: 200,
+             height: 75,
+             child: Stepper(
+               margin: margin,
+               type: StepperType.vertical,
+               steps: const <Step>[
+                 Step(
+                   title: Text('Regular title'),
+                   content: Text('Text content')
+                 ),
+               ],
+             ),
+           ),
+         ),
+       ),
+     );
+
+     final Stepper material = tester.firstWidget<Stepper>(
+       find.descendant(
+         of: find.byType(Material),
+         matching: find.byType(Stepper),
+       ),
+     );
+
+     expect(material.margin, equals(margin));
    });
 }