chore(ec2): add R8g instance type (#30803)

### Issue # (if applicable)

None

### Reason for this change

AWS EC2 now supports R8G instance type. But CDK L2 construct does not support this.  

### Description of changes

Update `InstanceClass`

### Description of how you validated changes

None

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-ec2/lib/instance-types.ts

@@ -376,6 +376,16 @@ export enum InstanceClass {
    */
   R7A = 'r7a',
 
+  /**
+   * Memory optimized instances with Graviton4 processors
+   */
+  MEMORY8_GRAVITON = 'memory8-graviton',
+
+  /**
+   * Memory optimized instances with Graviton4 processors
+   */
+  R8G = 'r8g',
+
   /**
    * Compute optimized instances, 3rd generation
    */
@@ -1466,6 +1476,8 @@ export class InstanceType {
       [InstanceClass.R7IZ]: 'r7iz',
       [InstanceClass.MEMORY7_AMD]: 'r7a',
       [InstanceClass.R7A]: 'r7a',
+      [InstanceClass.MEMORY8_GRAVITON]: 'r8g',
+      [InstanceClass.R8G]: 'r8g',
       [InstanceClass.COMPUTE3]: 'c3',
       [InstanceClass.C3]: 'c3',
       [InstanceClass.COMPUTE4]: 'c4',