Update S3 sdk to v2

[ianopolous]

Version 2 of the Amazon S3 sdk is now modularized - see aws/aws-sdk-go-v2#444

Updating to this should massively reduce the size of this plugin and may have performance benefits too. This would solve #81

[guseggert]

Note that there are still some sharp edges with v2, see e.g. aws/aws-sdk-go#5333