Programatically check configured AWS account has correct permissions on startup

[mdedetrich]

What is currently missing?

Currently when you use Guardian with the s3 backend it doesn't check if the configured S3 account has the necessary permissions for correct usage of Guardian.

If possible we can also check that the retention level for multipart upload chunks is greater than the configured time slice.

This is basically codifying what would be documented by #199

How could this be improved?

Current thinking is to use the official AWS Java S3 client to check ACL permissions (adding these features to Alpakka's S3 client would likely be considered out of scope and hence excessive). Just need to make sure that we use the same version of the Java S3 client that Alpakka brings in.

Is this a feature you would work on yourself?

• I plan to open a pull request for this feature