Minimal Support for Shared Config Loading (#675)

* create an AWS config module

This commit adds a new AWS config module. It only currently supports region resolution. A follow up commit will remove `ProvideRegion` from AWS types.

* create aws-config

This commit introduces `aws-config`, the configuration loader entry point for the SDK. Currently, this adds very little new code, instead, it migrates code from `aws_types` into the shared-config package.

* Update Examples to use shared-config

* Fix KMS IT

* Fix cargo docs

* Fix clippy

* Clarify doc comment on EnvLoader

* Clarify region chain impl, add tests

* Update CHANGELOG.md

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

* Delete dead module

* CR feedback

* Add more docs to aws-config

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