Add `serdes-descendants` for "containment" to serialize a subtree (#25017)

* Add `serdes-descendants` for "containment" to serialize a subtree

This allows naming eg. a Collection and will recursively serialize: all
dashboards, cards and dashcards it contains directly, plus all child
collections and everything they contain.

Currently this words on Collection, Dashboard, DashboardCard, and Card.

* lint

* Switch to plural `extract-subtrees` with a list of `targets`