Add support for the Smithy Auth trait (#652)

* Add support for the Smithy Auth trait

Some services have explicitly disabled authentication. This adds two things:
1. Customization to remove auth schemes for 2 STS operations
2. Add codegen support for the OptionalAuth and Auth trait to code generation.

CredentialsStage now will pass through unset credentials and the signer will check the signing trait.

A future enhancement may remove the signing middleware entirely.

* Update changelog

* Remove unused STS import

* remove unused variable

* Update CHANGELOG.md

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

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