Add vendored feature for dependency crates so that statically linked OpenSSL can be used

[yuroitaki]

Currently, it seems like when one uses features like tokio-native-tls, there is no option to use the vendored feature of its dependency crates, i.e. tokio-native-tls, native-tls, tungstenite, in order to use a statically linked copy of OpenSSL instead of dynamically link to the OS' OpenSSL (https://docs.rs/openssl/0.10.56/openssl/#vendored).

If the above assumption is true, can a feature similar to what's done in tokio-tungstenite be used to achieve this?

[sdroege]

Sure, do you want to provide a PR for that?