Add Lambda to update the WAFv2 IPSet

[kenpkzhang]

Issue #, if available:

Description of changes:
You could use WAF IPSet based rule to restrict the associated ALB or API Gateway to allow only traffic from the CloudFront IP range. This will reduce the attack surface from network layers if adversaries attempt to DDoS the ALB or API Gateway endpoints directly.

This Lambda function is to help you update the IPSet IP ranges automatically if CloudFront IP address ever changes using the AWS SNS topic. Set up of the solution can be refer to this blog post.
https://aws.amazon.com/blogs/security/how-to-automatically-update-your-security-groups-for-amazon-cloudfront-and-aws-waf-by-using-aws-lambda/

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[be-aws-architect]

Thank you for providing this, this helped me with implementing this for WAFv1 in my environment!