`aws-config`: Fix compilation error with `rustls` and `native-tls` disabled (#1533)

* `aws-config`: Fix compilation error with `rustls` and `native-tls` disabled

The `ProviderConfig::with_tcp_connector` method uses
`aws_smithy_client::hyper_ext`, which only exists with the
`client-hyper` feature enabled. Add a feature enabling that, and enable
it by default.

Introducing this feature does not cause breakage, because aws-config
did not previously compile with `default-features = false` and neither
`rustls` nor `native-tls` enabled.

* CHANGELOG.next.toml: Update for aws-config compilation fix

* Fix doctest by adding feature gate

Co-authored-by: Russell Cohen <rcoh@amazon.com>