This finishes the transition to the assume cache as source of truth for the
current set of claims.
The tests have to be adapted. It's not enough anymore to directly put objects
into the informer store because that doesn't change the assume cache
content. Instead, normal Create/Update calls and waiting for the cache update
are needed.
There's no reason for having the interface because there is only one
implementation. Makes the implementation of the test functions a bit
simpler (no casting). They are still stand-alone functions instead of methods
because they should not be considered part of the "normal" API.
This is now used by both the volumebinding and dynamicresources plugin, so
promoting it to a common helper package is better.
In terms of functionality, nothing was changed. Documentation got
updated (warns about storing locally modified objects, clarifies what the Get
parameters are). Code coverage should be a bit better than before (tested with
and without indexer, exercises event handlers, more error paths).
Checking for specific errors can now be done via errors.Is.
