Change graph workspace to store index pattern id instead of name

[flash1293]

Currently the graph-workspace saved object contains the name of the selected index pattern.
To be able to export and import them reliably (#34862), this should be changed to store the index pattern id instead (plus migration of saved workspaces).

To be able to do that, the migration has to support asynchronous steps (because it is necessary to look up the corresponding index pattern id during migration, issue for that is pending).

[elasticmachine]

Pinging @elastic/kibana-app

[mikecote]

Once this is completed, it will unblock #34997.

[timroes]

Blocked by #34996