Use OpenSSL in nix rust toolchain example

[AleksanderGondek]

This commit introduces the basic usage of openssl rust crate in nix rust toolchain example - which in turn, gives a good overview of steps required to make rules_rust + nix work with any packages which usually are delivered from the linux FHS.

Closes #207

[AleksanderGondek]

RE: CI errors fatal error: zlib.h: No such file or directory

I am unable to get any repro:

• VBox with Ubuntu 20.04 / 22.04 + nix 2.9.1 - PASS
• NixOS 22.05 + nix 2.8 - PASS

At this point I am a bit stumped.

[benradf]

This change has been merged in PR 305.