chore(ec2): add r8a instance class (#35964)

### Reason for this change
https://aws.amazon.com/about-aws/whats-new/2025/11/memory-optimized-amazon-ec2-r8a-instances/

### Description of changes
Add r8a instance class

### Description of how you validated changes
```console
$ aws ec2 describe-instance-types \
  --filters "Name=instance-type,Values=r8a.*" \ 
  --query "InstanceTypes[].InstanceType" \
  --output table
-----------------------
|DescribeInstanceTypes|
+---------------------+
|  r8a.large          |
|  r8a.2xlarge        |
|  r8a.medium         |
|  r8a.8xlarge        |
|  r8a.48xlarge       |
|  r8a.metal-24xl     |
|  r8a.metal-48xl     |
|  r8a.xlarge         |
|  r8a.12xlarge       |
|  r8a.24xlarge       |
|  r8a.16xlarge       |
|  r8a.4xlarge        |
+---------------------+
```

### 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*

Author: phuhung273

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

@@ -472,6 +472,16 @@ export enum InstanceClass {
    */
   R7A = 'r7a',
 
+  /**
+   * Memory optimized instances based on 5th generation AMD EPYC (formerly code named Turin), 8th generation
+   */
+  MEMORY8_AMD = 'memory8-amd',
+
+  /**
+   * Memory optimized instances based on 5th generation AMD EPYC (formerly code named Turin), 8th generation
+   */
+  R8A = 'r8a',
+
   /**
    * Memory optimized instances with Graviton4 processors
    */
@@ -1897,6 +1907,8 @@ export class InstanceType {
       [InstanceClass.R7IZ]: 'r7iz',
       [InstanceClass.MEMORY7_AMD]: 'r7a',
       [InstanceClass.R7A]: 'r7a',
+      [InstanceClass.MEMORY8_AMD]: 'r8a',
+      [InstanceClass.R8A]: 'r8a',
       [InstanceClass.MEMORY8_GRAVITON]: 'r8g',
       [InstanceClass.R8G]: 'r8g',
       [InstanceClass.MEMORY8_GRAVITON4_NVME_DRIVE]: 'r8gd',