Add rustls as a feature flag or use it by default.

[shekohex]

Is your feature request related to a problem? Please describe.
Currently, ethers-providers crate uses tokio-tungstenite with tls feature flag, which would use async-native-tls crate.
in Linux that means Openssl .. and you know what happens when you use Openssl and try to compile your cli/project to musl target!.

Describe the solution you'd like
I would like to have an option to enable the rustls-tls feature instead of the tls default one.
see: https://docs.rs/crate/tokio-tungstenite/0.14.0/features

Describe alternatives you've considered
Enable rustls-tls by default, which is more secure and more importantly easy to cross compile.

Additional context
CI Fails because of this: https://github.com/webb-tools/webb-rs/runs/2910394664?check_suite_focus=true

[gakonst]

@shekohex thanks for the issue! Could you check if #330 fixes it for you?

[shekohex]

Yes, that is awesome.
Thank you!