Annotation to manually set AWS_DEFAULT_REGION and AWS_REGION

[mattzech]

What would you like to be added:

We do not have our webhook configured to mutate Pods to include AWS_DEFAULT_REGION and AWS_REGION env vars by default.
Would it be possible to add an annotation on the ServiceAccount which can mutate the pods to include these env vars?

Why is this needed:

This would give a lot more freedom for users in our multi-tenanted cluster. We do not set these by default because some teams are performing actions in other regions. Let me know if this makes sense or you need more information. Thanks!