Implement S3-compatible storage for registry backend

[filiptibell]

This allows the registry backend to be used with other providers than GCP. We have been using this exact implementation in our organization without issues since January (https://github.com/readysetplay/wally/commit/1b4ceef242c6bc15055ebd9ab6e0c30b0ae0e52f) with hosting and storage on DigitalOcean.

S3 storage is configured using the following env variables:
AWS_REGION_NAME - Name of the storage region to use such as us-east-1
AWS_REGION_ENDPOINT - Full url endpoint of the S3 server. Some providers only need this and not the region name, so the region name can be omitted and defaults to the same region as the rusoto library that this implementation uses.

Authorization is done using S3 standards and utilities that the rusoto library provides.

The implementation is otherwise very similar to what is already implemented in Wally with GCP.

[magnalite]

These changes look good to me! I don't have a practical way to test if it works so I'm going to have to trust you or just wait for someone to test it in the real world and report any issues.

Seeing the above comment I think it would set a good precedent if we broke out requiring s3 into a feature flag!