chore: use rustls for reqwest

[lucacasonato]

This commit changes the auth library to use the rustls feature of reqwest instead of native-tls. This is done because rustls is already included as a dependency of google-cloud-auth anyway. There is no point in including multiple TLS libraries.

By extension this removes the dependency on OpenSSL.

This reduces the Cargo.lock file form 1217 lines to 1070 lines :-)