Minimum throughput body timeouts Pt.1 (#3068)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
https://github.com/awslabs/smithy-rs/issues/1562

## Description
<!--- Describe your changes in detail -->
This change adds a new body wrapper: The minimum throughput limit
wrapper. It tracks the rate that data is being streamed from itself. If
that rate falls below some configurable limit, it emits an error instead
of the next chunk. This protects users from requests that start quickly
but then slow down considerably.

I'd like to get this merged and then figure out the
codegen/docs/examples/config part in a separate PR.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
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Tests are included.

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