Allow overwriting of policy evaluationID by supplying new one in the evaluation request

This issue is a follow-up to issue #25 (closed), where we generate unique evaluationID which is used as cache key to store the results.

The new additional implementation should allow a client to supply his own "evaluationID" which will overwrite the generation of unique evaluationID.

This is currently on-hold until it's clarified with @SchulzeStTSI and @valerii.kalashnikov