Do not require 'key' and 'secret' options if AWS_WEB_IDENTITY_TOKEN_FILE is set

[datacticapertti]

This pull request enables use with EKS IAM Roles for Service Accounts https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts.html. In this use case, there is no underlying IAM user whose credentials are used. Instead, authorizations are passed in the environment variable AWS_WEB_IDENTITY_TOKEN_FILE.