RFC: Shared Config (#655)

* Add initial design for shared config

* Update shared config RFC

* add no-configuration proposal

* clarify `aws-types`, remove references to `aws-core`

* Update design/src/aws_config.md

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

* Update design/src/aws_config.md

Co-authored-by: David Barsky <me@davidbarsky.com>

* wip

* Update shared config RFC

* Update aws_config.md

* Update aws_config.md

* Update Shared config RFC to match reality

Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: David Barsky <me@davidbarsky.com>