[EOEPCA/IAM] Move OPA policies to separate repository

[w-jfe]

By now, OPA references a set of very general example policies provided with the Keykloak-OPA plugin.
It is expected that several EOEPCA-specific policies will evolve during the next few months. Therefore it makes sense to migrate to a new repository that should contain a copy of the example policies as well as new policies that are developed for EOEPCA.
The new repository may still be public, because it is meant as a standard set of policies for EOEPCA and also a template and starting point for policy sets of projects based on EOEPCA.