Add RFC for improving client error ergonomics (#2075)

## Motivation and Context
This RFC proposes merging `Error`/`ErrorKind` on code generated client
errors to simplify error handling.


[Rendered](https://github.com/awslabs/smithy-rs/blob/jdisanti-rfc-improve-error-ergonomics/design/src/rfcs/rfc0027_improve_client_error_ergonomics.md

)

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>