[RLC] Feature request: generate Known* enum for typespec extensible enum for RLC

[wanghoppe]

Currently in RLC generation, extensible enum will be generated as a string type, without the Known* enum type.

For example, TypeSpec definition:

RLC output

Also, just wonder if we can have a emitter options to have extensible enum generated as union type in Typescript? Like type CachingType = 'none' | 'read' | 'readwrite', will this proposal make it possible? microsoft/typespec#3363