Re-export ByteStream's Length and FsBuilder in client crates

[ysaito1001]

Motivation and Context

awslabs/aws-sdk-rust#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

• 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.
• 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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[landonxjames]

LGTM!