Use namespace index if possible when encoding ExpandedNodeId

In OpcUaBinaryEncoder::encodeExpandedNodeId, if we know the namespace index, then use that when encoding instead of a URI.

It's more compact than encoding a namespace.

It's also a workaround for bugs observed in the CTT and UaExpert, where the Client ignores the explicitly specified namespace URI in the ExpandedNodeId and just uses the namespace index, which is always 0 when the URI is encoded, instead.