Add support for `native-tls` when using websocket transport

[ondrowan]

This PR adds support for native-tls when using Transport::Wss.

This is still a WIP since the feature tokio-native-tls needs to be set in async-tungstenite when using use-native-tls feature in this crate. At the same time, if use-rustls is set in this crate, async-tungstenite needs to set feature tokio-rustls-native-certs. I have no idea how to edit Cargo.toml to support both of these cases.

At the moment async-tungstenite always uses - even if using insecure WS connection - tokio-rustls-native-certs feature, which I don't think is correct either.

Any help with this or other things I've missed in this PR would be appreciated.

Type of change

New feature (non-breaking change which adds functionality)

Checklist:

• [x] Formatted with cargo fmt
• [ ] Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.