Support change to SiteWhitelist/SiteBlacklist in ReqMgr2 for active workflows

[amaltaro]

Impact of the new feature
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
With this ticket, we are supposed to implement changes to ReqMgr2 such that SiteWhitelist and SiteBlacklist are allowed to be updated in workflows that are in one of the given statuses:

staging, acquired, running-open

Note that this change needs to be persisted in two places:

• workflow high level description (json)
• workflow spec file (the WMWorkload object file)

If possible, we should also validate whether the sitelists is actually changing or not. If none of them change, then nothing should be persisted in the database and no extra actions need to be triggered.

The reason we are not going to accept these changes for workflows in assigned and staged is such that we can guarantee no data race between services dealing with the same workflow.

An initial point for this development will have to be made here:
https://github.com/dmwm/WMCore/blob/master/src/python/WMCore/ReqMgr/DataStructs/RequestStatus.py#L118

Describe alternatives you've considered
Unless explicitly requested, I don't think we need to add these capabilities to the ReqMgr2 UI.

Additional context
None