fix(aws-ec2): Add Burstable Generation 3 Instances (#812)

Add T3/Burstable3 to the instance types enumeration.

Author: leepa

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

@@ -128,6 +128,16 @@ export enum InstanceClass {
    */
   T2 = 't2',
 
+  /**
+   * Burstable instances, 3rd generation
+   */
+  Burstable3 = 't3',
+
+  /**
+   * Burstable instances, 3rd generation
+   */
+  T3 = 't3',
+
   /**
    * Memory-intensive instances, 1st generation
    */