Add WebIdentityProvider to AWS credentials chain

[benfu-verses]

I am trying to run Alexandrie in Kubernetes but running into the issue where the default credentials chain does not use the injected IAM Roles for Service Account credential. The upstream issue is here: rusoto/rusoto#1781

Could we add a custom chain provider that just includes the rusoto_sts::WebIdentityProvider::from_k8s_env() in the chain? Something like awslabs/coldsnap#122 (comment).

Alternatively, switching to the AWS Rust SDK would also fix this behavior by default.