Support loading S3 credentials from shared credentials file

[brandond]

K3s currently supports loading etcd s3 credentials from 3 places:

• Config file/cli flags
• S3 config secret
• Instance metadata service (IAM)

We should add support for also loading credentials from a shared credentials file (~/.aws/credentials): https://docs.aws.amazon.com/cli/latest/topic/config-vars.html#the-shared-credentials-file

This would make it possible to use AWS_SHARED_CREDENTIALS_FILE and AWS_PROFILE env vars to load credentials from an existing file on disk, in addition to loading credentials from the default file and profile if neither are set.

Ref:

• https://github.com/minio/minio-go/blob/master/pkg/credentials/file_aws_credentials.go#L65-L67
• https://github.com/minio/minio-go/blob/master/pkg/credentials/chain.go#L20-L44