Add send_with method to fluent builders (#2652)

## Motivation and Context
This is a child PR of https://github.com/awslabs/smithy-rs/pull/2615.

## Description
- Adds `send_with` method to Fluent Builder.

## Prerequisite PRs
You can merge this first too reduce diffs.

- https://github.com/awslabs/smithy-rs/pull/2651



## Testing
NA

## Checklist
NA

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Co-authored-by: John DiSanti <johndisanti@gmail.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>