feat(ec2): add r6i instances (#17663)

New R6I instances just got released:

https://aws.amazon.com/about-aws/whats-new/2021/11/amazon-ec2-r6i-instances/

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

@@ -118,6 +118,16 @@ export enum InstanceClass {
    */
   R5 = 'r5',
 
+  /**
+   * Memory optimized instances, 6th generation with Intel Xeon Scalable processors (3rd generation processors code named Ice Lake)
+   */
+  MEMORY6_INTEL = 'r6i',
+
+  /**
+   * Memory optimized instances, 6th generation with Intel Xeon Scalable processors (3rd generation processors code named Ice Lake)
+   */
+  R6I = 'r6i',
+
   /**
    * Memory optimized instances for high performance computing, 5th generation
    */

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

@@ -138,7 +138,7 @@ describe('instance', () => {
   });
   test('instance architecture is correctly discerned for x86-64 instance', () => {
     // GIVEN
-    const sampleInstanceClasses = ['c5', 'm5ad', 'r5n', 'm6', 't3a']; // A sample of x86-64 instance classes
+    const sampleInstanceClasses = ['c5', 'm5ad', 'r5n', 'm6', 't3a', 'r6i']; // A sample of x86-64 instance classes
 
     for (const instanceClass of sampleInstanceClasses) {
       // WHEN