expose connector in user agent (#3667)

## 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 -->
part of the `hyper` 1.0 upgrade

## Description
<!--- Describe your changes in detail -->
This PR adds information about the connector used to send a request to
the metadata included in the user agent.

## Testing
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to -->
<!--- see how your change affects other areas of the code, etc. -->
I added an integration test.

## Checklist
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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

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