Support for alternative TLS backends

[dignifiedquire]

I am currently looking at alternatives for quinn, but due to my current usage, I am relying on rustls for TLS handling (platform support, and custom certificate verification). So I was wondering if you are open to enabling different TLS backends. Either in a fully generic form, or by feature flagging boringssl & rustls.

[dignifiedquire]

Adding support for rustls, would likely make supporting windows #44 easier as well.

[iyangsj]

Our current design encapsulates the dependency on the TLS library. We did this on purpose so we can easily switch to or add other TLS libraries later on.
We're thinking about adding rustls to later versions to take advantage of Rust's memory safety.