feat(#2072237): allow disabling File (Field) Paths processing site-wide and per entity

Add a site-wide "enabled" setting (config + settings form) and a transient filefield_paths_settings entity property so migrations and other programmatic saves can suppress or override processing for a single save without touching field configuration. Includes an update hook to default existing sites to enabled and a runtime requirements warning when disabled site-wide.

Issue #2072237 by jose reyero, herved, trebormc, feng-shui, rudolfbyker, geek-merlin, vistree, deciphered.