fix(elbv2): explicitly implement IApplicationTargetGroup (#1806)

Fix usage of application and network target groups in languages
without structural typing, such as Java and C#.

Fixes #1799.

Author: rix0rrr

packages/@aws-cdk/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts

@@ -62,7 +62,7 @@ export interface ApplicationTargetGroupProps extends BaseTargetGroupProps {
 /**
  * Define an Application Target Group
  */
-export class ApplicationTargetGroup extends TargetGroupBase {
+export class ApplicationTargetGroup extends TargetGroupBase implements IApplicationTargetGroup {
   /**
    * Import an existing target group
    */

packages/@aws-cdk/aws-elasticloadbalancingv2/lib/nlb/network-target-group.ts

@@ -34,7 +34,7 @@ export interface NetworkTargetGroupProps extends BaseTargetGroupProps {
 /**
  * Define a Network Target Group
  */
-export class NetworkTargetGroup extends TargetGroupBase {
+export class NetworkTargetGroup extends TargetGroupBase implements INetworkTargetGroup {
   /**
    * Import an existing listener
    */