Support for `rustls` not only `openssl`

[gabrik]

Hi, first thank you for your useful crate.

I was thinking if it is possible to add a couple of features to decide whether to use reqwest with openssl or rustls, this will allow to build this crate and the tool without any extenal depedencies especially for systems where openssl may not be present.

Please take a look at the features of reqwest to check how it can be done: https://docs.rs/crate/reqwest/latest/features