feat(aws-smithy-types): Impl http-body-1.0 Body for SdkBody (#3380)

## Motivation and Context
A step towards moving to http-1.0
https://github.com/awslabs/aws-sdk-rust/issues/1046



(Russell): This is a minimal implementation of `http-body = 1`. It isn't
maximally efficient since even if we were given a 1x body, we convert it
back and forth first. This is a first step.

## Description
Implements http-body-1.0 Body trait for SdkBody.

## Testing
Regular CI

## 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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Co-authored-by: Russell Cohen <rcoh@amazon.com>
Co-authored-by: Russell Cohen <russell.r.cohen@gmail.com>
Co-authored-by: John DiSanti <john@vinylsquid.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>