Fix C# + C++ SDKs for making Options filterable and expand sdk-test

[JasonAtClockwork]

Description of Changes

• Is acompanion to Make Options filterable on clients, and add SDK tests. #5448 expanding the tests to TS/C#/C++
• Updated sdk-test-ts, sdk-test-cs, and sdk-test-cpp to match Rust version; a couple changes were necessary to allow the codegen to work:
  • Updated C++ table constraint helpers to support std::optional<T> as filterable
  • Updated C# filterability checks to handle nullable values for IsEquatable()
• Updated C# regression tests to cover nullable lookups, this uncovered more changes required:
  • Updated C# codegen to use the new nullable index base classes instead of removing constraints that Nullable<T> doesn't support
• Updated C++/Unreal generated bindings to support the new Option<T> index tables and reducers used by the SDK tests

API and ABI breaking changes

• Adds generated C# nullable index accessor support for indexes that were already intended to be filterable

Expected complexity level and risk

3 - Mostly test coverage and generated binding updates, with small changes to allow this properly in C++ and C#

Testing

• Ran full sdk-test suite
• Ran the C# regression tests
• Manual testing of C# nullable index throwaway e2e test
• Ran full Unreal SDK tests

[gefjon]

I don't feel entirely qualified to review the implementation, but the tests look reasonable, and that's enough to convince me. It looks like you've missed committing some generated files, as the git tree check part of CI is failing. Otherwise, if you want to wait for @lisandroct 's review, I support that, but if you want to merge, go ahead.