The `shards_availability` indicator diagnoses the condition where
indices need to be restored from snapshot.
Starting with 8.0 using feature_states when restoring from snapshot is
mandatory.
This adds support for the `FEATURE_STATE` affected resource to aid with
building up the snapshot restore API call (which will need to include
all the indices and feature states reported by the restore-from-snapshot
diagnosis).
Note that the health API will not report any indices that are part of a
feature state.
