`aws-config` uses sync file IO in an async context

The `aws-config` crate is currently using sync file IO in the async credential loading path: https://github.com/awslabs/smithy-rs/blob/main/aws/rust-runtime/aws-types/src/os_shim_internal.rs#L128

This should be changed to async IO, but that presents a challenge since to be async runtime agnostic, the async spawn implementation will need to be configurable.

A configurable async spawn should be added to `aws-smithy-async`, and made configurable through `SdkConfig` and `ConfigLoader`. Then the file IO operations can be fixed.

Start in aws/rust-runtime/aws-types/src/os_shim_internal.rs at the cited credential-loading path, then inspect aws-smithy-async, SdkConfig, and ConfigLoader for existing async configuration and spawning patterns. The work is done when async file IO is used without assuming a specific runtime and the async spawn implementation can be configured through the SDK configuration path.

Written by the indexing model from the issue text.