AWS OIDC authentication support for running s3-loader in EKS

[jalkjaer]

This PR adds support for using the OIDC based authentication. This is used with running the container in an EKS cluster with IAM roles associated to K8S Service accounts (see https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html)

To enable the accessKey and secretKey should be set to oidc

    aws {
      accessKey = "oidc"
      secretKey = "oidc"
    }

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[jalkjaer]

I signed it!

[snowplowcla]

Confirmed! @jalkjaer has signed the Contributor License Agreement. Thanks so much.

[svdimchenko]

hi there! any updates about this PR ? I faced same problem and this PR has fixed it. Many thanks for author!