feat(ec2): add im4gn and is4gen instances (#17780)

Newly announced `im4gn` and `is4gen` storage optimized instances:
https://aws.amazon.com/blogs/aws/new-storage-optimized-amazon-ec2-instances-im4gn-and-is4gen-powered-by-aws-graviton2-processors/

![image](https://user-images.githubusercontent.com/31543/144113879-5451ce52-eeb1-4ad2-ba80-8ac3d67ce533.png)

![image](https://user-images.githubusercontent.com/31543/144113921-b71f67a7-a1c6-425d-9dc6-4f0eed03c08e.png)

CFN docs have already been updated:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html#cfn-ec2-instance-instancetype

----

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

Author: robertd

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

@@ -442,6 +442,26 @@ export enum InstanceClass {
    */
   I3EN = 'i3en',
 
+  /**
+   * Storage optimized instances powered by Graviton2 processor, 4th generation
+   */
+  STORAGE4_GRAVITON_NETWORK_OPTIMIZED = 'im4gn',
+
+  /**
+   * Storage optimized instances powered by Graviton2 processor, 4th generation
+   */
+  IM4GN = 'im4gn',
+
+  /**
+   * Storage optimized instances powered by Graviton2 processor, 4th generation
+   */
+  STORAGE4_GRAVITON_NETWORK_STORAGE_OPTIMIZED = 'is4gen',
+
+  /**
+   * Storage optimized instances powered by Graviton2 processor, 4th generation
+   */
+  IS4GEN = 'is4gen',
+
   /**
    * Burstable instances, 2nd generation
    */

packages/@aws-cdk/aws-ec2/test/instance.test.ts

@@ -122,7 +122,7 @@ describe('instance', () => {
   test('instance architecture is correctly discerned for arm instances', () => {
     // GIVEN
     const sampleInstanceClasses = [
-      'a1', 't4g', 'c6g', 'c6gd', 'c6gn', 'm6g', 'm6gd', 'r6g', 'r6gd', 'g5g', // current Graviton-based instance classes
+      'a1', 't4g', 'c6g', 'c6gd', 'c6gn', 'm6g', 'm6gd', 'r6g', 'r6gd', 'g5g', 'im4gn', 'is4gen', // current Graviton-based instance classes
       'a13', 't11g', 'y10ng', 'z11ngd', // theoretical future Graviton-based instance classes
     ];