add: example showing how to use native-tls (#946)

* add: native-tls example and test
update: aws-config to correctly separate the native-tls and rustls features for its dependencies
fix: native-tls feature gate typo
update: prefix unused id field with an underscore

* update: CHANGELOG.next.toml

* remove: unnecessary aws-sdk-sts feature deps from aws-config

* remove: obsolete dep features