RFC for forwards compatible errors (#3108)

## Motivation and Context
This RFC describes a way to allow services to add model previously
unmodeled variants in the future without breaking customers.

## Description

[Rendered](https://github.com/awslabs/smithy-rs/blob/errors-forward-compat-rfc/design/src/rfcs/rfc0039_forward_compatible_errors.md

)

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