chore(ec2): warn if egress rules are ignored (#9127)

If `allowAllOutbound` is set to true, any call to `addEgressRule` will be ignored, this PR adds a warning. 

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packages/@aws-cdk/aws-ec2/lib/security-group.ts

@@ -404,6 +404,7 @@ export class SecurityGroup extends SecurityGroupBase {
       // In the case of "allowAllOutbound", we don't add any more rules. There
       // is only one rule which allows all traffic and that subsumes any other
       // rule.
+      this.node.addWarning('Ignoring Egress rule since \'allowAllOutbound\' is set to true; To add customize rules, set allowAllOutbound=false on the SecurityGroup');
       return;
     } else {
       // Otherwise, if the bogus rule exists we can now remove it because the