Allow Filtering/Dropping Unsupported Data Types

[SharkBaitDLS]

Right now, there are datatypes such as BigDecimal (see #312) that are not yet supported for codegen. To ease friction for clients that want to build off models that include these datatypes, but don't necessarily need them for their Rust use-case, an opt-in configuration that allows dropping those fields during codegen instead of failing with an exception would offer a workaround until those datatypes can be bottomed out with a long-term solution.