[Design]: Support Discriminated Unions in all client emitters

### Clear and concise description of the problem
Unions can be marked as discriminated using the `@discriminated` decorator. however, all the existing emitters haven't support this features.
We need to support Discriminated Unions

This issue is created to track all the design discussion to support Discriminated Unions

cc @tadelesh

It would be good if we could have a loop to enable people to comment directly.
TCGC discussion could be found in: https://github.com/microsoft/typespec/issues/8455