Review FileSystemRealm XML Types

Ideally if we can achieve a type system that allows older clients to still make use of the entry for an identity even if some aspects of the later config it does not understand.

The filesystem realm is complicated further in that the older version may wish to make updates so we would also need to consider some form of generically persisting the XML not understood but at the same time we don't know if the portion not understood relates to the current update. The later version may also need to compensate for this in some way.