Integrate with simulate policies API

[simonw]

https://www.reddit.com/r/aws/comments/qlu3ag/comment/hj68pmv/ pointed out simulate_custom_policy() which is an API method that lets you try out custom policies. https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/iam.html#IAM.Client.simulate_custom_policy

Must be some useful stuff I can do with this. A CLI command for testing policies with it would be fantastic.