Allow using Rustls instead of OpenSSL

[drakon64]

Allow using Rustls instead of OpenSSL. The default is to continue to use reqwest's default-tls feature so this should not affect existing consumers.

Rustls can be opted-into with the rustls-tls feature flag. Other reqwest Rustls options can be enabled with the same feature flags that reqwest accepts.

Fixes #45.

[FlixCoder]

The actual solution would be to:

• disable default features on reqwest
• add feature flags like "default-tls", "native-tls", "rustls", "rustls-native-roots", "http2", etc. and forward them to reqwest via native-tls = ["reqwest?/native-tls"].
• add "default-tls" to the default features

As explanation/reasoning: cargo feature flags should be additive, i.e. enabling two features may not conflict.

[drakon64]

@FlixCoder Is e02ca52 what you were expecting? I left the other reqwest defaults in-place to avoid this being a breaking change.

[FlixCoder]

Yes, thanks.

I would personally add even more feature flags (for http2, charset, http3, compressions, various more tls/tokio settings, ...) and the default needs some macos feature flag apparently. or maybe default-tls becomes reqwest-default and enables reqwest default features to avoid future breaking changes.
Then everything is covered :D

But if you don't want to do that it is fine, I would merge it and adjust ir afterwards :)

Feel free to disable draft mode ^^

[drakon64]

HTTP/3 support requires an additional compiler flag, so I'd rather leave that one out.

Cookies require an opt-in on the ClientBuilder so more work would be needed than just enabling the feature.

I've added all of the reqwest flags that seemed relevant.