Include system data streams when snapshotting feature states

[williamrandolph]

In the course of work on #75819, we discovered that we have not connected system data streams and snapshots. If a user creates a snapshot for a specific set of indices and the Fleet feature state, the fleet data stream and backing indices will not be included. This is a bug, though at the moment it only affects Fleet. (A general snapshot and restore of the cluster will include all feature states.)

We need to add system data stream handling to the snapshot and restore code for feature states.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)