Query the backend details for the compile-time feature in libgit2.

This will return the "backend" for the feature, which is useful for things like HTTPS or SSH support, that can have multiple backends that could be compiled in.

For example, when libgit2 is compiled with dynamic OpenSSL support, the feature backend will be openssl-dynamic. The feature backend names reflect the compilation options specified to the build system (though in all lower case). The backend may be "builtin" for features that are provided by libgit2 itself.

If the feature is not supported by the library, this API returns NULL.