Feat: Allow Recommendations to explain why a particular assignment is made

[triceo]

The current recommended fit API serves the purpose of finding the best possible spot for a newly created (not yet assigned) entity. Once the spot is found, it can explain why it picked this spot, and not some other. However, this behavior is not enough. In order to be able to explain a solution to humans, we need to have the same explanation capabilities also for entities that have already been assigned previously.

To this end, we will:

• Allow initialized entities to enter the recommendations API as well.
• Return recommendations which compare not against an uninitialized solution, but rather against the fully initialized solution provided.
• Deprecate the recommendFit endpoint and replace it with recommendAssignment in a backwards compatible manner. The behavior of the endpoint will not change for uninitialized solutions.