Enabling ALB Support

[atrope]

Description of changes:
This is a non breaking change, already tested and working in a production environment.

The ALB lambda integration is not well documented..

I Had to remove the Access-Control-Allow-Credentials header when the request is coming from an ALB because it was returning 502 when I tried with that header.

Also i found out(via test and errors) that the order of the returned object does matter as isBase64Encoded must be before the body, if it is not.

This should save the user the $3.5 per 1M if he does not need more features from APIGW, and it does also works with Cloudfront.

With this PR it will continue to work with APIGW without changes.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[atrope]

Why is this PR not being merged? it would save users lots of $ in High load scenarios..
I Am using this in Prod with 10M MAU, so it is "battle tested"

[beomseoklee]

Thanks for your contribution, @atrope
We've added this feature to v5.1.0. Unfortunately, due to the internal policy, we don't merge the PR directly. You can see the detail in CHANGELOG and your name in README.

Once again, thanks for your contribution.