[FEATURE] Support virtual fields in sources

[vpmv]

Unchained features a lot of configurability. Are you sure the feature is not part of existing configuration?

• I have read the full wiki

Is your feature request related to a problem? Please describe.
Currenlty, only "physical" columns are collected in Junctions. It would be great to allow virtual fields (e.g. pointers) as well.

Describe the solution you'd like
Select virtual fields in my source and seeing them on dashboards

Additional context

# app-members.yaml
name:
   type: text
surname:
   type: text
surname_abbr:
   type: text
   pointer: 
      fields: [surname]
   ignored: true
   _transform:
        text: { abbr: true }

# app-events.yaml
participants:
    source:
        source: members
        fields: [name, surname_abbr]