feat: add rustls-webpki feature

[QuietMisdreavus]

Closes #15

This PR adds a new Cargo feature: rustls-webpki. This allows users to use both the rustls TLS stack, as well as the webpki-roots static CA certificates, to enable a truly independent implementation.

The basic idea is that now there are four primary versions of the library:

1. No TLS support (building with no features)
2. TLS via native-tls (default, using the tls feature)
3. TLS via rustls and native CA certificates (using the rustls feature)
4. TLS via rustls and compiled-in CA certificates from webpki-roots (using the rustls-webpki feature)

The two rustls features build off a new rustls-base feature, which sets up the common code between rustls and rustls-webpki. The two user-facing features then configure the TlsConnector in ProxyConnector::new based on their respective certificate store crates.

One question i have about the initial implementation: Since i introduced a separate cargo feature (rustls-base) to represent the common rustls code, there's a potential situation where someone deliberately sets the rustls-base feature without setting either rustls or rustls-webpki to add certificates to the TLS configuration. This will create a situation where it looks like you can set up a TLS connection to the proxy, but nothing will succeed because there are no certificates loaded into the trust store for the connection. Should we set up a guard in the library code to emit a compile_error!() macro if this happens?

Thanks for setting up this library! I'm looking forward to integrating it with my own. 😁

[tafia]

This is awesome!
Would you mind updating the readme with the same comment as the PR reagrding all the features?
Regarding rustls-base, I believe that documenting it is enough.

[QuietMisdreavus]

I've added a new section to the readme about the features, and added a comment to the Cargo.toml about rustls-base. What do you think?

[tafia]

Awesome
Thanks! I'll publish ASAP