Server API : add callback for node availability on create MonitoredItem
Add a callback to allow application to notify a node might be available in the future and create MonitoredItem should succeed for this known node. This allows the server in combination with AddNodes service (#965 (closed)) to create dynamically nodes "on demand", i.e. when client requests to monitor this node, and the client will be notified once the value becomes available.
This is allowed by specification of Create Monitored Item service (part 4) through this sentence:
If a NodeId is known to be valid by a Server but the corresponding Node Attributes are currently not available, the Server may allow the creation of a MonitoredItem and return an appropriate Bad StatusCode in the Publish response.
Edited by Vincent Monfort