implement http-body 1.0 for PathBody (#3673)

## 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 -->
#1925 

## Description
<!--- Describe your changes in detail -->
Implements the v1 `http_body::Body` trait for `PathBody`. Part of the
ongoing hyper v1 upgrade.
This also moves a pre-1.0 impl into its own module.

## Testing
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<!--- Include details of your testing environment, and the tests you ran
to -->
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I ported the tests too

## Checklist
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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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