feat(ec2): add g5g instances (#17765)

New `g5g` instances release notes:
https://aws.amazon.com/blogs/aws/new-amazon-ec2-g5g-instances-powered-by-aws-graviton2-processors-and-nvidia-t4g-tensor-core-gpus/

![image](https://user-images.githubusercontent.com/31543/143961493-528f208d-f941-43d8-ba23-7f1d6b813913.png)


Docs have already been updated a while ago:
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*

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

@@ -486,6 +486,16 @@ export enum InstanceClass {
    */
   G5 = 'g5',
 
+  /**
+   * Graphics-optimized instances powered by AWS Graviton2 Processors and NVIDIA T4G Tensor Core GPUs, 5th generation
+   */
+  GRAPHICS5_GRAVITON2 = 'g5g',
+
+  /**
+  * Graphics-optimized instances powered by AWS Graviton2 Processors and NVIDIA T4G Tensor Core GPUs, 5th generation
+  */
+  G5G = 'g5g',
+
   /**
    * Parallel-processing optimized 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', // current Graviton-based instance classes
+      'a1', 't4g', 'c6g', 'c6gd', 'c6gn', 'm6g', 'm6gd', 'r6g', 'r6gd', 'g5g', // current Graviton-based instance classes
       'a13', 't11g', 'y10ng', 'z11ngd', // theoretical future Graviton-based instance classes
     ];