Allow filtering by "simple" C-style enums (sums where all variants have unit as the payload)

[bfops]

Will need updates to:

• Rust module macro, to derive FilterableValue for these types.
• C# source generators.
  • Hopefully this is sufficient for both the C# client SDK and the C# module library.

[bfops]

seems like a followup to https://github.com/clockworklabs/SpacetimeDB/issues/2368?

[bfops]

(dup of above issue)