Add ability to list "active" backfills both across cluster and per-dag (UI-only API -- not public)

[dstandish]

Body

child of parent issue #43970

UI needs the ability to fetch the active backfill for a dag, and the list of all active backfills across the cluster.

Add this only in the UI part of the core API, so that it is not considered "public" and therefore we can change it as desired without having to wait until airflow 4.0

i.e. here:

cc @jedcunningham @bbovenzi @phanikumv @pierrejeambrun

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[vatsrahul1001]

cc: @prabhusneha assigning this to you.

[prabhusneha]

cc: @prabhusneha assigning this to you.

Sure, will take this up.

[phanikumv]

Weird that @prabhusneha is getting unassigned automatically, and that is showing up as @vatsrahul1001 unassigned her, seems to be a problem in GitHub. However it seems to be sorted now