Split previews out of the filecache table

[CarlSchwan]

The oc_filecache table can grow quite a bit, and a large part of this is due to the preview table.

Currently we store the previews in this table because it allows us to make use of all the different storage abstractions, we uses for normal files. This should be possible to change to only use IStorage or IObjectStore as abstraction while moving the data out of the filecache into a more optimized table

@AndyScherzinger

[artonge]

Follow-up:

• [x] https://github.com/nextcloud/server/pull/54543#discussion_r2414212908
• [ ] Consolidate migrations for large instances