Support EKS Pod Identity credentials (#3416)

## Motivation and Context
I would like to support EKS Pod Identity credentials in the Rust SDKs

## Description
This brings the ECS provider in line with other sdks (eg, Go) by
supporting AWS_CONTAINER_AUTHORIZATION_TOKEN_FILE as well as permitting
http IPs to be non-loopback if they are the EKS pod identity IPs.

## Testing
I have added various new unit tests, and I have updated the existing
integration test to also create pods with eks pod identity creds, which
I have used to test in a real EKS cluster as well.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Signed-off-by: Jack Kleeman <jackkleeman@gmail.com>
Co-authored-by: John DiSanti <john@vinylsquid.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>