Add CredentialsProvider to config (and related refactorings) (#182)

* Add credentials provider config & test

* Prepare to use `Features` during Operation Generation

* Fix sdk gradle build changes that were previously missed

* Utilize the credentials provider in the feature

* Update IdempotencyTokenProvider to be Send + Sync

* Add accessors for credential providers

* Fix build.gradle

* Update auth references to be aws-auth

* Update idempotency token test