Feature: #59830 - Introduce read-only column for file mounts

See Issue #59830

Description

File mount records now have a new flag "read only". This flag replaces the virtual flag introduced earlier, so it can be defined natively in the record.

Impact

The impact is low as the old behavior still exists. A storage was never marked as read-only before. There is an option to set this through UserTs, but now it is also possible to set it on the storage record directly.