Add support for Rustls

[Lorak-mmk]

Some time ago Rust Driver gained support for Rustls as backend for TLS.
It makes sense to add support for it in cpp-rust-driver. The main benefit is not requiring a library to be installed on the system, so building and using the driver becomes easier.
It remains an open question how exactly should we support it.
Imo there should definitely be a new CMake switch, similarly to how we now have one for openssl.
Should backend be chosen at runtime (as in Rust Driver), or at compile time? How exactly should the API look like? I don't know.

[dkropachev]

Having multiple binaries for both backends will be a problem.
What if we have a switche in cmake that define which TLS backend gets into binary and have environment variable that controls which one is used.
So that we could have generic binary that have both on board and, at the same time, have ability to build with only one backend on board.