feat(dgw): dynamically load XMF native lib on startup

[CBenoit]

By default, we look for the library at /usr/lib/libxmf.so on Linux, and xmf.dll alongside the executable on Windows.

Developers may specify a different path to xmf.dll / libxmf.so using lib_xmf_path debug option:

{
  "__debug__": {
    "lib_xmf_path": "/usr/local/lib/libxmf.so"
  }
}