chore(sagemaker): added support for ml.g6 instances (#30948)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

G6 instances were announced for general availability in april 2024

[Announcing general availability of Amazon EC2 G6 instances](https://aws.amazon.com/about-aws/whats-new/2024/04/general-availability-amazon-ec2-g6-instances/)
[G6 instances can be used for sagemaker notebooks and endpoints](https://aws.amazon.com/about-aws/whats-new/2024/05/amazon-sagemaker-notebooks-g6-instance-types/)

![image](https://github.com/user-attachments/assets/ca212a55-df2b-49a8-9183-a87adda2a715)

I would like to add the instance type in cdk so that cdk app can spin up g6 instance as needed

### Description of changes

defined new instanceTypes following the same convention as G5 instances

### Description of how you validated changes

minor change and existing file does not have unit tests

### 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/aws-sagemaker-alpha/lib/instance-type.ts

@@ -204,6 +204,46 @@ export class InstanceType {
    */
   public static readonly G5_XLARGE = InstanceType.of('ml.g5.xlarge');
 
+  /**
+   * ml.g6.12xlarge
+   */
+  public static readonly G6_12XLARGE = InstanceType.of('ml.g6.12xlarge');
+
+  /**
+   * ml.g6.16xlarge
+   */
+  public static readonly G6_16XLARGE = InstanceType.of('ml.g6.16xlarge');
+
+  /**
+   * ml.g6.24xlarge
+   */
+  public static readonly G6_24XLARGE = InstanceType.of('ml.g6.24xlarge');
+
+  /**
+   * ml.g6.2xlarge
+   */
+  public static readonly G6_2XLARGE = InstanceType.of('ml.g6.2xlarge');
+
+  /**
+   * ml.g6.48xlarge
+   */
+  public static readonly G6_48XLARGE = InstanceType.of('ml.g6.48xlarge');
+
+  /**
+   * ml.g6.4xlarge
+   */
+  public static readonly G6_4XLARGE = InstanceType.of('ml.g6.4xlarge');
+
+  /**
+   * ml.g6.8xlarge
+   */
+  public static readonly G6_8XLARGE = InstanceType.of('ml.g6.8xlarge');
+
+  /**
+   * ml.g6.xlarge
+   */
+  public static readonly G6_XLARGE = InstanceType.of('ml.g6.xlarge');
+
   /**
    * ml.inf1.24xlarge
    */