Make rustls as a default (or perhaps drop native-tls)

[hatoo]

native-tls block when calling https://docs.rs/tokio-native-tls/latest/tokio_native_tls/native_tls/struct.TlsConnectorBuilder.html#method.build

Sometimes this leads to a problem

❯ cargo run --release -- -n 1 -c 10 --http2 --insecure --no-tui  https://localhost:8090/
   Compiling oha v0.6.5 (/home/hatoo/oha)
    Finished release [optimized] target(s) in 6.67s
     Running `target/release/oha -n 1 -c 10 --http2 --insecure --no-tui 'https://localhost:8090/'`
Summary:
  Success rate: 100.00%
  Total:        2.7290 secs # <- too long

Whereas rustls works well

❯ cargo run --no-default-features --features rustls  --release -- -n 1 -c 10 --http2 --insecure --no-tui  https://localhost:8090/
   Compiling oha v0.6.5 (/home/hatoo/oha)
    Finished release [optimized] target(s) in 6.81s
     Running `target/release/oha -n 1 -c 10 --http2 --insecure --no-tui 'https://localhost:8090/'`
Summary:
  Success rate: 100.00%
  Total:        0.0086 secs

We can mitigate this by playing with tokio but ... I don't want to.