Allow inspection data to be gathered in events, not just by API calls

[andchiind]

Describe the new feature you would like to see
Currently we collect and upload inspection data by making a function call to robotinterface, which in turn return the inspection data in a synchronous way. It is natural for many robots to process the inspection data asynchronously and therefore return the inspection data in an event based manner. I would like ISAR to support such a setup.

Describe the solution you'd like
There should be a config option that sets whether inspection data is gathered by calling the "get_inspections" function in robotinterface, or if ISAR should use an event handler which can be registered using a new function in robotinterface. ISAR should use this event handler to automatically upload inspection data as soon as it is received.

This should not be a breaking change as long as the current inspection data uploading is kept as the default in settings.py.

How will this feature affect the current Threat Model?
N/A, as the connections used for the events are handled in the respective ISAR implementations.