Optional body parameter is handled differently in HLC and Modular which would cause breakings

[MaryGao]

Optional body will be put in optional params in HLC but in Modular it would be top-level parameter in method with optional.

see hlc swagger: https://github.com/Azure/autorest.typescript/blob/main/packages/autorest.typescript/test/integration/swaggers/appconfiguration.json#L457-L462
hlc generated code: https://github.com/Azure/autorest.typescript/blob/main/packages/autorest.typescript/test/integration/generated/appconfiguration/src/models/index.ts#L512-L513

modular generated code:
https://github.com/Azure/autorest.typescript/blob/main/packages/typespec-ts/test/modularIntegration/generated/parameters/body-optionality/src/index.d.ts#L25-L26

[MaryGao]

Offline confirmed with Qiaoqiao, we think this is an un-necessary breaking and we should avoid it. I plan to fix in pr.