Emitter needs to adopt TCGC's recent changes on nullable

[chunyu3]

TCGC will label Nullable on property/parameter instead of type.
Codegen emitter will adapt this:

• update emitted model, apply Nullable on property/parameter
• update inputModel in autorest.csharp accordingly.
• Still label Nullable in CsharpType, update the TypeFactory.cs to pass Nullable parameter when create CsharpType.

[ArcturusZhang]

we could take this opportunity to review and refine the design of our input types as well

[ArcturusZhang]

Also this should fix #4314

[archerzz]

Latest decision is to deprecate all nullable, and return union for nullable types in TCGC. There will be a helper function isNullable to help check if the type is nullable. For details, see Azure/typespec-azure#870