Add `Credentials` to `aws-config` (#678)

* Add ProvideCredentials to aws-types

* Integrate ProvideCredentials into SDK runtime and codegen

* Add CredentialsChainProvider to aws-config

* Move lazy_caching and async_provide_credentials_fn to config

* Migrate profile & web identity token providers

* Complete refactoring of aws_auth_providers

* Update examples

* Delete profile parser from aws-types and move fuzz tests

* Cleanups

* Fix some test failures

* Fix docs

* Fix select-object-content example

* convert tests, remove STS

* Fix doc comment

* remove usages of test-env-log

* take 2 fix doc comment

* Update changelog

* rename asycn_provide_credentials_fn