feat(ec2): add X2gd instances (#16810)

Add support for X2gd instances.

Announcement: https://aws.amazon.com/about-aws/whats-new/2021/03/announcing-new-amazon-ec2-x2gd-instances-powered-by-aws-graviton2-processors/

Closes #16794.

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*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-ec2/lib/instance-types.ts

@@ -372,6 +372,16 @@ export enum InstanceClass {
    */
   X1E = 'x1e',
 
+  /**
+   * Memory-intensive instances, 2nd generation with Graviton2 processors and local NVME drive
+   */
+  MEMORY_INTENSIVE_2_GRAVITON2_NVME_DRIVE = 'x2gd',
+
+  /**
+   * Memory-intensive instances, 2nd generation with Graviton2 processors and local NVME drive
+   */
+  X2GD = 'x2gd',
+
   /**
    * Instances with customizable hardware acceleration, 1st generation
    */