Create `CACHEDIR.TAG` in `appdata_xxx/preview` for easy exclusion from backups

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Is your feature request related to a problem? Please describe.

Preview data directories can grow to be quite large (mine is 356GB) but have no need to be backed up since all their data can be regenerated at-will. Nextcloud should give backup tools some help avoiding storing a bunch of unnecessary cache data.

Describe the solution you'd like

CACHEDIR.TAG is a de-facto standard for indicating that a directory contains only cached data, with wide support (for example, tar, BorgBackup, Restic, and Duplicati all support this standard).

Nextcloud should automatically write CACHEDIR.TAG, with the correct magic contents, to appdata_xxx/preview/CACHEDIR.TAG upon creation of the parent directory. (At least, it should do this when the data storage driver is the local filesystem and not e.g. object storage.)

Describe alternatives you've considered

Document this directory as one that should probably be excluded from backups.

Additional context

Search the PHP code that creates the appdata_xxx/preview directory and inspect how the local filesystem storage driver is identified. Add the standard CACHEDIR.TAG with its required magic contents when that directory is created, excluding object storage, and verify that the tag appears in the intended preview path.