Missing smithy-trait-codegen union support

[msosnicki]

We're using the new smithy-trait-codegen module to generate the java trait files for the protocols. Seem that the unions are not supported yet https://github.com/smithy-lang/smithy/blob/1805e515bc3877a02aa8351da1a7ea62e92a2c8f/smithy-trait-codegen/src/main/java/software/amazon/smithy/traitcodegen/TraitCodegenSymbolProvider.java#L184.

Is it something on the radar for the upcoming releases? If not, would contribution speed up the process?

Thanks