[codegen] support optional fields with different conditions

Currently the mechanism we use to support optional fields only supports versions, and assumes that a higher version of a table always contains all the fields of previous versions.

This breaks with post, where 3.0 has fewer fields than 2.0. In addition, there have been a few other instances where we have something like this; for instance in lookup subtables, the `markFilteringSet` exists conditionally on the appropriate bit being set in the lookup flags.

Overall this would give us more flexibility in codegen, and save us hand-writing code.

> That's actually an interesting complication for codegen, this is the first time where we will have a lower version of a table (2.0) that has more fields than a higher version (3.0). We make a bunch of assumptions about fields only being added with versions, so I'll have to think about this some.

_Originally posted by @cmyr in https://github.com/googlefonts/fontations/issues/576#issuecomment-1680955670_