MSPileup: update documents in MongoDB with the customName attribute

[amaltaro]

Impact of the new feature
MSPileup

Is your feature request related to a problem? Please describe.
To be done after or in parallel to: #11734

Once we converge into the new field name, we need to ensure that existing documents get updated accordingly, in addition to updating our tools and the current pileup configuration dump that we have.

Describe the solution you'd like
The following tasks are expected from this ticket:

• fetch all the MSPileup documents from MongoDB and update them to have the customName field as well
  • this has to be done for integration and production (perhaps for development databases as well?)
• this script needs to be updated accordingly: https://github.com/dmwm/WMCore/blob/master/bin/adhoc-scripts/createPileupObjects.py
• these 2 pileup dumps need to be updated as well: https://github.com/dmwm/WMCore/tree/master/test/data/WMCore/MicroService/DataStructs

Describe alternatives you've considered
None

Additional context
This is part of the meta issue: #11537