Better detection and error messages for indexes with unsupported key types

Currently, the UX of restricted index key types is pretty poor. You don't get a compile-time error when creating the index, you just don't have access to the filtering methods. The client SDKs also inconsistently or incorrectly do/don't emit client-side indexes when the module declares an index of unsupported type.

We've seen a few different users in the public Discord stumble on this.

Definition of Done:

• In the Rust module bindings library, it is a hard compile-time error to declare a BTree, unique or primary_key index on (a) column(s) which do(es) not implement FilterableValue.
• In the C# module bindings library, it is a hard compile-time error to declare a BTree, unique or primary_key index on (a) column(s) whose type(s) is/are not "filterable," i.e. the same set of types as in Rust implement FilterableValue.
• In the Rust client SDK, it is possible to spacetime generate bindings for a module with an index whose types are not filterable. The resulting bindings compile and run without error, but do not contain the ill-typed index. That is, spacetime generate for the Rust client SDK skips indexes with non-filterable key types. A warning is printed to the console when skipping such an index during spacetime generate.
  • This should be unreachable due to the above restrictions, but I expect we'll encounter modules that bypass those restrictions in various ways.
• In the TypeScript client SDK, it is possible to spacetime generate bindings for a module with an index whose types are not filterable. The resulting bindings compile and run without error, but do not contain the ill-typed index. That is, spacetime generate for the TypeScript client SDK skips indexes with non-filterable key types. A warning is printed to the console when skipping such an index during spacetime generate.
• In the C#/Unity Client SDK, it is possible to spacetime generate bindings for a module with an index whose types are not filterable. The resulting bindings compile and run without error, but do not contain the ill-typed index. That is, spacetime generate for the Rust client SDK skips indexes with non-filterable key types. A warning is printed to the console when skipping such an index during spacetime generate.

Trace index validation in the Rust and C# module bindings, then follow index handling in spacetime generate for the Rust, TypeScript, and C#/Unity client SDKs. Compare how FilterableValue and filterable types are represented, then use the definition of done to verify compile-time errors, skipped indexes, warnings, and successfully generated bindings.