Add a RuntimePlugin/Interceptor to enforce expected content length (#3491)

## 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 -->
There is a rarely-triggered bug
(https://github.com/awslabs/aws-sdk-rust/issues/1079) that can occur
when the runtime is dropped between requests. Although this is
definitely the _wrong thing to do_(tm) which should still aim to at
least protect the users from bad data in this case.

This adds an interceptor which validates that the body returned is the
correct length.

## Description
- Adds an interceptor that computes the actual content length of the
body and compares it to the actual length

## Testing
- Integration style test. Note that this is very hard to test using
Hyper because Hyper will attempt to mitigate this issue.


## 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


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