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

[jdisanti]

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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.