Support Ditto searches comparing values of the twin data

[thjaeckle]

There are scenarios where one would not like to define a concrete value to search for, e.g.:

lt(features/heartbeat/properties/lastReceiveTime,time:now-2h)

But instead compare 2 (or more) values in the twin, e.g. (in this example, relying on that nextExpectedReceiveTime is updated with each lastReceiveTime dynamically to the next expected "heartbeat" message from a device):

lt(features/heartbeat/properties/lastReceiveTime,features/heartbeat/properties/nextExpectedReceiveTime)

I found that the MongoDB $expr might be able to do such queries:
Compare Two Fields from a Single Document

We would need to:

• make sure that $expr really works for such a use case
• define how the RQL queries could look like, targeting a dynamic value from the twin
• enhance Ditto's search service to be able to map to BSON
• enhance Ditto's ThingPredicateVisitor to also support this (e.g. in order to be used within condition)