## Motivation and Context
https://github.com/awslabs/aws-sdk-rust/issues/820

## Description
This PR re-exports `ByteStream`'s `Length` and `FsBuilder`. By making
these types available in a client crate, customers can use
`ByteStream::read_from` without needing to import them separately from
the `aws-smithy-types` crate.

## Testing
- Updated an existing re-export test to include `Length` and `FsBuilder`
as well.
- Existing tests in CI

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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