TSP, TCGC adoption, use `SdkServiceMethod` rather than `SdkHttpOperation` to generate method signature

[haolingdong-msft]

[weidongxu-microsoft]

Part of the support done in microsoft/typespec#5898

However, the implementation separates "flatten" and "group".

Ideally, there would be one process on sdkMethod.parameters and httpOperation.parameters, and generate the full mapping of flatten+group in code-model.

[weidongxu-microsoft]

The remaining is more on design.

1. What kind of method is valid, e.g.
   op op1(@bodyRoot body: { @bodyRoot body1: {}}): void; likely not
2. Whether change of behavior affect existing code (potential breaking changes)? How many would be affected? Can these break be migitated?
3. Whether service really want this signature (where most of it is determined by template, not what service write)?