OpenSSL in a separate library

[vinniefalco]

I would like to explore putting all of the OpenSSL-specific bits into a separately compiled lib, e.g. libbeast2_ssl.so or whatever.

If necessary we can put everything in include/boost/beast2/ssl and src/ssl to make it easier to isolate the files but lets make sure that the second lib is going to be viable. The goal here is to avoid the need for configuration macros although we do want to be able to know inside the CMakeLists.txt, the examples, and in translation units whether or not the build system found OpenSSL. By defining BOOST_BEAST2_OPENSSL_AVAILABLE perhaps. We have to make sure that we dont use this to create multiple versions of the same lib.