Improve signing scheme mapping from std::string to OpenSSL function pointers

[SylvainCorlay]

xeus dynamically maps the signing scheme specified in the kernel messages to the corresponding function pointer in OpenSSL.

This is done in function asevp, at https://github.com/QuantStack/xeus/blob/0.23.3/src/xauthentication.cpp#L106-L124.

This function internally makes use of a std::map with std::string keys and const EVP_MD*(*)() (function pointers).

Unfortunately, depending of the build options of OpenSSL, some of these functions may or may not be defined, so we had to comment out some of the supported schemes.

We should investigate if this information can be discovered at runtime.