Support S3 access points (and multi-region access points)

[rocketbitz]

Currently if I specify a valid access point URL such as: s3::https://mskgi7fk7dnbi.mrap.accesspoint.s3-global.amazonaws.com/some/path/to/module.tgz I get the following error:

│ Could not download module "module" (main.tf:5) source code from "s3::https://mskgi7fk7dnbi.mrap.accesspoint.s3-global.amazonaws.com/some/path/to/module.tgz":
│ InvalidParameter: 2 validation error(s) found.
│ - minimum field size of 1, GetObjectInput.Bucket.
│ - minimum field size of 1, GetObjectInput.Key.

Ideally if the session is authenticated, it should be able to pull objects through APs (and MRAPs) in the same way that it does directly from S3 buckets.

[crw]

Likely would be improved by #467.