Issue #3069511: Replace existing files option for File (Field) Paths

Reroll onto current 8.x-1.x. Adds a 'Replace existing files' setting; in replace mode the move() re-uses the managed file already at the destination, so the field is re-pointed at the returned file and persisted with a nested save (fresh original so core does not skip the field as unchanged, updateLoadedRevisionId + setNewRevision(FALSE) so it is valid mid-save). Includes a same-name upload functional test. Note: the nested-save block reads/writes $entity->original directly; happy to switch to getOriginal()/setOriginal() with DeprecationHelper if preferred for 11.2+.