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. [x] Decorate brownfield services with @bodyRoot
2. [x] Update emitter logic
3. [x] 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

• [X] Follow our Code of Conduct
• [X] 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
• [X] Read the docs.
• [X] 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