pub fn from_k8s_env() -> Self {
        Self::_from_k8s_env(
            Variable::from_env_var(AWS_WEB_IDENTITY_TOKEN_FILE),
            Variable::from_env_var(AWS_ROLE_ARN),
            Variable::from_env_var_optional(AWS_ROLE_SESSION_NAME),
        )
    }
Creat a WebIdentityProvider from the following environment variables:

- `AWS_WEB_IDENTITY_TOKEN_FILE` path to the web identity token file.
- `AWS_ROLE_ARN` ARN of the role to assume.
- `AWS_ROLE_SESSION_NAME` (optional) name applied to the assume-role session.

See https://docs.aws.amazon.com/eks/latest/userguide/iam-roles-for-service-accounts-technical-overview.html
for more information about how IAM Roles for Kubernetes Service Accounts works.
rusoto/services/sts/src/custom/web_identity.rs