Re-export `aws_smithy_types::date_time::Format` in service client crates (#2534)

* Add more client crate re-exports

This commit adds two more client re-exports that were not included
in smithy-rs#2437:
- `aws_smithy_http::byte_stream::Length`
- `aws_smithy_types::date_time::Format`

* Remove re-export of `aws_smithy_http::byte_stream::Length`

This commit removes re-export of `aws_smithy_http::byte_stream::Length`
for now as it is a feature-gated type, which requires further consideration.

* CHANGELOG.next.toml

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Co-authored-by: Yuki Saito <awsaito@amazon.com>