Rust client SDK codegen: skip and warn for ill-typed indexes.

[gefjon]

In the Rust client SDK, it should be possible to spacetime generate bindings for a module with an index whose types are not filterable. The resulting bindings should compile and run without error, but should not contain the ill-typed index. That is, spacetime generate for the Rust client SDK skips indexes with non-filterable key types. A warning should be printed to the console when skipping such an index during spacetime generate.

This should be unreachable after #2494 and #2495 , but I expect we'll encounter modules that bypass those restrictions in various ways.