Allow compiling against OpenSSL on macOS

We're kind of hit by the issue of Apple's Secure Transport being super picky about certificates since macOS Catalina [in Tiberius](https://github.com/prisma/tiberius/issues/65). We really can't change how Microsoft creates their certificates or what kind of certs Apple accepts. I was thinking could it be possible to force this crate to statically compile against OpenSSL, so we could distribute a version of Prisma for macOS that could talk TLS with SQL Server?

I don't know is this the right solution here, but that's how node.js solved the issues for their users: statically linking to OpenSSL on all platforms.

The issue names no files, tests, or entry points. Start by tracing the crate's current macOS TLS backend and how compilation features select it, then assess the feasibility of static OpenSSL linking and distribution. Done means a documented, reproducible macOS build can use OpenSSL and establish TLS with SQL Server.

Written by the indexing model from the issue text.