feat(ec2): add c8gn instance class (#34866)

### Issue # (if applicable)
None

### Reason for this change
https://aws.amazon.com/about-aws/whats-new/2025/06/amazon-ec2-c8gn-instance/

### Description of changes
EC2 add c8gn instance class

### Describe any new or updated permissions being added




### Description of how you validated changes
```console
$ aws ec2 describe-instance-types | grep -e c8gn
            "InstanceType": "c8gn.4xlarge",
            "InstanceType": "c8gn.16xlarge",
            "InstanceType": "c8gn.8xlarge",
            "InstanceType": "c8gn.12xlarge",
            "InstanceType": "c8gn.xlarge",
            "InstanceType": "c8gn.48xlarge",
            "InstanceType": "c8gn.2xlarge",
            "InstanceType": "c8gn.metal-24xl",
            "InstanceType": "c8gn.large",
            "InstanceType": "c8gn.medium",
            "InstanceType": "c8gn.24xlarge",
            "InstanceType": "c8gn.metal-48xl",
```

### 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

@@ -674,6 +674,18 @@ export enum InstanceClass {
    */
   C7GN = 'c7gn',
 
+  /**
+   * Compute optimized instances for high performance computing, 8th generation with Graviton4 processors
+   * and high network bandwidth capabilities
+   */
+  COMPUTE8_GRAVITON4_HIGH_NETWORK_BANDWIDTH = 'compute8-graviton4-high-network-bandwidth',
+
+  /**
+   * Compute optimized instances for high performance computing, 8th generation with Graviton4 processors
+   * and high network bandwidth capabilities
+   */
+  C8GN = 'c8gn',
+
   /**
    * Compute optimized instances based on Intel Xeon Scalable (Sapphire Rapids) processors, 7th generation
    */
@@ -1827,6 +1839,8 @@ export class InstanceType {
       [InstanceClass.C8G]: 'c8g',
       [InstanceClass.COMPUTE8_GRAVITON4_NVME_DRIVE]: 'c8gd',
       [InstanceClass.C8GD]: 'c8gd',
+      [InstanceClass.COMPUTE8_GRAVITON4_HIGH_NETWORK_BANDWIDTH]: 'c8gn',
+      [InstanceClass.C8GN]: 'c8gn',
       [InstanceClass.STORAGE2]: 'd2',
       [InstanceClass.D2]: 'd2',
       [InstanceClass.STORAGE3]: 'd3',