'created_at' timestamp for new STA objects

I just had a thought working on the ntr-code-resolver. We should add a timestamp to the properties of Things Datastreams Sensors when we create the in a STA endpoint.

In theory, the propertis.code should be unique for each entity. However, this is not guaranteed and we might end up with duplicate Datastrems, Things & Sensors, e.g. due to race conditions on entity creation.

If we record the creation timestamp, we can always return the oldest entity with a given code, making the behaviour deterministic. De-duplication of entities could thus be less urgent.

What says you @marc-adolf @sylvia-reissmann ?