Auto-detect whether OPC-UA support is present and use it if possible

Recent Fanuc controllers/system software versions could have OPC-UA support.

As much of the functionality in `dominh` could be implemented using that, it would be good for `dominh` to try and use it when possible.

But as `dominh` should remain usable with controllers/system software versions which do not have OPC-UA, it should remain optional.
