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

[joshtriplett]

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.

Motivation and Context

Without this change:

error[E0433]: failed to resolve: could not find `hyper_ext` in `aws_smithy_client`
   --> /home/josh/.cargo/registry/src/github.com-1ecc6299db9ec823/aws-config-0.15.0/src/provider_config.rs:251:50
    |
251 |             let mut builder = aws_smithy_client::hyper_ext::Adapter::builder()
    |                                                  ^^^^^^^^^ could not find `hyper_ext` in `aws_smithy_client`

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[joshtriplett]

Just realized I missed the generated-code note in Cargo.toml here (and there wasn't one in the .rs file). Will send to smithy-rs instead.