Enable crypto library flexibility

[bifurcation]

Right now, things are hard-wired to OpenSSL -- OpenSSL 1.1, even. It would be nice to upgrade to the OpenSSL 3 series, or maybe support mBedTLS, WolfSSL, etc. I expect the way to do this is probably roughly as libsrtp has, with uniform interface and a .cpp file per back-end (see, e.g., all the aes_gcm_${LIB}.c files in the crypto/cipher diretory). Then you can switch between them via the build system.

It may be that the existing crypto.cpp is isolated enough that it's the right target. At a minimum, you would want to do things like rename openssl_error to crypto_library_error and make sure there were no OpenSSL dependencies outside of crypto.cpp.

[bifurcation]

Fixed in #58