This PR adds the ability of chaining the credentials provider,
such that restic as a tool attempts to honor credentials from
multiple different ways.
Currently supported mechanisms are
- static (user-provided)
- IAM profile (only valid inside configured ec2 instances)
- Standard AWS envs (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY)
- Standard Minio envs (MINIO_ACCESS_KEY, MINIO_SECRET_KEY)
Refer https://github.com/restic/restic/issues/1341