* feat: add new Folder table migration
Add a new folder table to support the Nested Folders feature.
https://github.com/grafana/grafana/issues/56880
* register nested folders feature flag (unused)
* feat: nested folder service (experiment)
This commit adds a NestedFolderSvc interface and stubbed out implementation as an alternative to the existing folder service. This is an experimental feature to try out different methods for backwards compatibility and parallelization, so that Grafana can continue to store folders in the existing (non-nested) manner while also using the new nested folder service.
Eventually the new service will (hopefully) become _the_ service, at which point the legacy service can be deprecated (or remain, with the new service methods replacing the original. whatever makes sense at the time).
* nested folders: don't run the new migration
This commit removes the nested folder migration from the list of active migrations so we can merge this branch and continue development without impacting Grafana instances built off main.
* Chore: move folder service interface into a separate package
* copy implementation into a standalone package
* move implementation and tests to the new folder package
* remove leftovers from wire
* add test doubles for folder service
* fix tests in library panels/elements
* fix provideservice in ngalert
