MSPileup: support custom container DID in the doc schema

[amaltaro]

Impact of the new feature
MSPileup

Is your feature request related to a problem? Please describe.
This ticket is only relevant if we decide to adopt the alternative described in this ticket:
#11621

which would leverage the fact that we can create custom Rucio DID containers and attach rucio datasets to it, whenever the pileup container fraction deviates from 1.0

Describe the solution you'd like
If we adopt custom pileup container DIDs, it means we need to start persisting that new DID in the MSPileup backend database.

That said, this ticket requires the following development:

• modify the MSPileup object data structure to support another string value, e.g. customName. Its default value is supposed to be an empty string and it has to be populated whenever a custom Rucio container is created

Whenever customName is not an empty string, it needs to be used in lieu of the pileupName attribute. Thus tasks like:

• fetching the pileup size
• fetching container level rule ids
• tracking rules and updating data location
  needs to be performed against the customName attribute.

Note that Rucio DID needs to follow lexicon rules. A simple rule that we could follow is to append -WMA to the original container name. However, if we prefer not to change the length of the container name, we could replace the last 4 characters by -WMA.

Describe alternatives you've considered
None

Additional context
This ticket requires the creation of a new scope in Rucio. As chatted with Eric V. today, it can be done and he suggested in setting a group scope, e.g. "group.wma".