Impact of the new feature
Global WorkQueue (and ReqMgr2)

Is your feature request related to a problem? Please describe.
This is a sub-task of this meta-issue: #8323
Towards providing a feature that allows workflow site lists to be changed while workflows are active in the system.

Describe the solution you'd like
Whenever an user updates either the SiteWhitelist or the SiteBlacklist attributes of a workflow, ReqMg2 needs to:

• load the workflow GQE
• filter elements that are in status Available
• update the site lists for each element

Please double check whether the list of sites in WQE is actually a list of sites, and not locations (RSEs). If the latter, then we also need to remap sites to RSEs.

If a failure happens during this process, nothing should be persisted in the workflow spec and a clear error message needs to be returned to the end user.

Describe alternatives you've considered
None

Additional context
Depends on #12037

For GQE statuses, please refer to: https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/WorkQueue/DataStructs/WorkQueueElement.py