re-enable content length enforcement (#3618)

## Motivation and Context
This PR re-enables content-length runtime plugin that was previously
disabled due to some erroneous tests. Those
[tests](https://github.com/awslabs/aws-sdk-rust/blob/main/sdk/dynamodb/tests/retries-with-client-rate-limiting.rs#L29)
appear to be fixed.
 
Original PR: https://github.com/smithy-lang/smithy-rs/pull/3491
Tracking issue: https://github.com/smithy-lang/smithy-rs/issues/3523


## 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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