Extract common functionalities of the auditors of an entity into abstract parent class

[twwd]

In the most (all?) auditors, we start the audit with looping over all entities, check whether they are ignored and then perform the actual check. We can extract the common parts into a abstract base class per entity (e..g., client, idp) like it's done for realm auditors with #41.
We might even find a common abstract base class for all entity types using generics but that probably leads to over-engineering.