docs: Add AWS_IAM authorization example

[myrkytyn]

Description

Adds documentation and a working example for using authorization_type = "AWS_IAM" with API Gateway HTTP APIs. This includes:

• A new route example in examples/complete-http/main.tf demonstrating AWS_IAM authorization
• Updated main README.md with a concise AWS_IAM usage example

Motivation and Context

Closes #86

I was struggling to understand how to configure AWS_IAM authorization for API Gateway routes. The existing documentation didn't include any examples of this authorization type, making it unclear how to set up the route with authorization_type IAM.

Breaking Changes

None - this is a documentation-only enhancement.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request