Add support for IAM Authorizers in combination with 2.0 payload

[jonife]

Issue #, if available:
#780

Description of changes:
Add IAM authorizers in addition to existing JWT, Lambda authorizers
see also https://docs.aws.amazon.com/lambda/latest/dg/urls-invocation.html

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[deki]

Great work, thank you very much for your contribution @jonife.