Support canned policies and fully support custom policies

[oliversalzburg]

Contrary to how the policy file in this project is named, canned policies are not supported. Canned policy usage does not produce a Policy parameter, only custom policies do that.

For reference: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-creating-signed-url-canned-policy.html

Similarly, fully supporting custom policies would also be nice. DateGreaterThan is currently not supported.