[sequoia-net] Feature for using rustls instead of openssl

Depending on sequoia-net uses openssl by default, via its hickory* dependencies, which makes it tricky to build for certain platforms (e.g. older Android versions). Plus, openssl may be something to avoid for other reasons.

A feature for making sure only rustls gets used in sequoia-net would be welcome.

The following manual patch picks rustls over openssl, and the tests run and pass (on macOS).

net/Cargo.toml:

hickory-client = { version = "0.24", features = ["dns-over-rustls"] }
hickory-resolver = { version = "0.24", features = ["rustls"]}