Spread is spread (parameters)

[iscai-msft]

Clear and concise description of the problem

We no longer want to recover a model from spread by default. The current behavior creates a complicated story for spreading, and leads to confusing emitter logic. The following needs to be done in this order

1. Decorate brownfield services with @bodyRoot
2. Update emitter logic
3. Add overrideClientMethod decorator
4. Move vendored BodyParameter to core

The final step is not required for the story to be complete, I'm only linking it in this issue as a reference.

Down the line, we also want to look into clearing up the logic in responses too. We will wait for a traits redesign first.

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[weidongxu-microsoft]

typespec-java 0.18.0 released with this logic

[iscai-msft]

Closing because all of the tcgc work has been completed with the introduction of the @override decorator