fix use_fips in provider config (#3007)

I'm not 100% that I fixed this in the right way. Feel free to set me
straight if that's the case.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
aws-sdk-rust#882

## Description
<!--- Describe your changes in detail -->
This change causes the`ProviderConfig` to respect both `use_fips` and
`use_dual_stack` when those settings are configured in a user's
environment or profile.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
I wrote two tests

## 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
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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choice._

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Co-authored-by: John DiSanti <jdisanti@amazon.com>