Feature: #67228 - Emit Signal when an IndexRecord is marked as missing

See Issue #67228

Description

The new signal recordMarkedAsMissing is emitted when the FAL indexer encounters a sys_file record which does not have a corresponding filesystem entry and marks it as missing. It passes the sys_file record uid.

Impact

This can be used by extensions that provide or extend file management capabilities (versioning, synchronizations, recovery etc).