Add a provider to support EKS Pod Identity

[ry4n1m3]

This PR adds a credential provider that can acquire credentials using the EKS Pod Identity system. This enables authenticating to AWS services from Kubernetes pods running in AWS EKS.

Licensing:
This contribution is made by employees of Mechanical Orchard, Inc. under the terms of the project’s license.

[onno-vos-dev]

LGTM

Linter is unhappy though due to tabs in aws_credentials_eks could you fix that up? If no response is given by Sunday, I will push a fixup instead 👍 I remember discussing EKS with an old colleague before but it never led anywhere since they ended up not using EKS so this is a really nice addition 🙇 Thank you!

Once merged, I can follow up and bump deps across aws-elixir and aws-erlang if you like? 👍

[slackersoft]

Got the formatting, but had a bit of a fight with dialyzer over types. I ended up expanding the value side of header() in aws_credentials_httpc to declare binary() | string() which is what it looks like httpc accepts under the hood.

[onno-vos-dev]

Released as 0.3.5 thank you both! 🙇 ❤️