Enhancement: Allow AWS Assume Role to be used for S3 backend

Previously only credentials discovered via the Minio Click discovery methods
would be used to authenticate. However there are many circumstances where the
discovered credentials have lower permissions and need to assume a specific role.

New Environment Variables:

- RESTIC_AWS_ASSUME_ROLE_ARN
- RESTIC_AWS_ASSUME_ROLE_SESSION_NAME
- RESTIC_AWS_ASSUME_ROLE_EXTERNAL_ID
- RESTIC_AWS_ASSUME_ROLE_REGION (if need to override from us-east-1)
- RESTIC_AWS_ASSUME_ROLE_POLICY
- RESTIC_AWS_ASSUME_ROLE_STS_ENDPOINT