Add an RFC for limiting the HTTP public dependency (#2899)

## Motivation and Context

[Rendered](https://github.com/awslabs/smithy-rs/blob/d3eaf5eea561af90146b7aa80da0307d49f63506/design/src/rfcs/rfc0036_http_dep_elimination.md

)

## Description
An RFC describing the need and proposed path for limiting the scope of
our HTTP dependency.

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choice._

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