Add conversions from smithy StatusCode to http StatusCode. (#3637)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
Making this conversion bidirectional is helpful.
<!--- If it fixes an open issue, please link to the issue here -->

## Description
<!--- Describe your changes in detail -->
Add `impl From<StatusCode> for httpN::StatusCode` that does the obvious
thing.

## 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'm using these conversions in my own project. As all three crates have
identical constraints on the status code value the changes are trivially
correct.

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

Not clear to me what if anything to do here.

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