FR: CSG parser to ignore unknown data

PLR files have been converted into .json, and one of their features is that the parser doesn't upchuck whenever it finds a field it doesn't know about. This makes the PLR files naturally forward compatible and allows newer pilot saves from being lost or corrupted whenever a player tries to load the same pilot with different versions of FSO.

The CSG can't do that currently, because its a binary file and has a strict ordering that it expects its data to be in. Every time there's a new field to be added to the CSG it will break compatibility, and is overall a nuisance.

Some tweaks to the CSG parser and maybe the file layout itself should allow for unknown data fields.