Create mock class/data for MSPileup

[amaltaro]

Impact of the new feature
WMCore unit tests

Is your feature request related to a problem? Please describe.
With a general migration of pileup data location from Rucio to MSPileup - which still resolves it via Rucio - in multiple layers/services of the WMCore, we started seeing failing unit tests here and there, as the previous location was retrieved through mocked Rucio.

A few examples of those unit tests are:

WMCore_t.WorkQueue_t.WorkQueue_t.WorkQueueTest:testProcessingWithPileup changed from success to failure
WMCore_t.WorkQueue_t.Policy_t.Start_t.Block_t.BlockTestCase:testPileupData changed from success to failure
WMCore_t.WorkQueue_t.WorkQueue_t.WorkQueueTest:testPileupOnProduction changed from success to failure

Describe the solution you'd like
In order to have reliable unit tests, which provide a better code coverage, we should ideally mock calls made to MSPileup, mostly through this module:
https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/Services/MSPileup/MSPileupUtils.py

This mocking setup includes:

• have the actual data to be served within unit tests
• instead of triggering an HTTP call to MSPileup, to use a mocked class (function?) and data

Describe alternatives you've considered
Another alternative would be to patch each required unit test, but I am not sure we can patch a sole function.

Additional context
This can be part of the meta-issue to provide partial pileup: #11537

Pull requests that can be impacting this functionality - and/or providing a transient workaround - are:
#11879
and
#11870