Make native TLS optional, and add a rustls option

[LoganDark]

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Right now, digitalocean pulls in reqwest with its default features. This includes a requirement of being linked to native OpenSSL. I'm cross-compiling my Rust application so it can be deployed onto a Droplet without Rust installed, and not everybody is able to just compile OpenSSL like I did (even I had some issues). Perhaps their cross-compilation toolchain is incomplete, or they just don't want to go through the trouble, and want a fast setup.

I'd like you to consider adopting reqwest's convention of having native-tls and rustls-tls features, with native-tls being the default, so that can be disabled and replaced by rustls-tls. I'm not sure how possible this is (does cargo even have this capability?) though.

[Hoverbear]

We talked last night and it seems like we might do this via removing reqwest for something else?