Enhance subscribing for change notifications by optional filter

[thjaeckle]

When currently subscribing for Events/change notifications via

• WebSocket
• SSE
• Connectivity

the consumer always gets all change notifications it is allowed to see.

On SSE this can be reduced by providing specific thingIds and fields so that changes are only published if the thingId match or a change was affected by a specified field.

The subscription should and could be more fine-grained.
The idea is to support adding an optional filter defined via Ditto's RQL syntax it already uses for the search.

That way the following subscription rules could be applied:

• notify me of a change only if the thingId starts with org.eclipse.ditto:* : like(thingId,"org.eclipse.ditto:*")
• notify me of a change only if the feature temperature was affected by this change: exists(feature/temperature)
• notify me of a change only if in one change the temperature was greater 25: gt(feature/temperature/properties/value,25)
• notify me of changes in the namespace org.eclipse.ditto:* affecting the temperature: and(like(thingId,"org.eclipse.ditto:*"),exists(feature/temperature))