Add support for AWS_WEB_IDENTITY_TOKEN_FILE (AWS EKS IRSA)

[jessedye]

Currently I have need for support of AWS_WEB_IDENTITY_TOKEN_FILE along with the AWS_ROLE_ARN with to fetch temporary accesskey and secret keys from Amazon STS using the minio-js SDK. This is due to my current k8s implementation using both minio and IRSA for S3.

Relevant Links: https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html

[prakashsvmx]

minio-js does not have extensive list of credential providers. feel free to send a PR @jessedye