Remove transitive dependency on OpenSSL in reqwest

[renecouto]

Hi, thanks for your rust package!
Since this package uses Rustls, we should be able to remove the openssl transitive dependency, as it makes bundling a bit harder (need to install libssl, openssl...). Can I open a PR set the "features" property on the reqwest dependency as to remove the dependency on openssl?

example: seanmonstar/reqwest#495

[kenkoooo]

Thank you for your suggestion! Yes, we definitely want to include feature flags to allow users to choose between Rustls or native TLS.