[Multipart] Support model format for `@multipartBody`

fixes part of https://github.com/Azure/typespec-azure/issues/960
Pending on https://github.com/microsoft/typespec/pull/3676

Context: Typespec supports 3 kinds of format to define multipart:

common

op upload( @header content-type: "multipart/form-data", @body body: { basic: string, headShots: bytes[], } ): void;


2. advanced model format

op upload( @header content-type: "multipart/form-data", @multipartBody body: { fullName: HttpPart, headShots: HttpPart<bytes[]> } ): void;


3. advanced array format

op upload( @header content-type: "multipart/form-data", @multipartBody body: [ // single HttpPart<string, #{ name: "fullName" }>, HttpPart<bytes[], #{ name: "headShots" }>, ] ): void;


- This PR is implementation for API design https://github.com/Azure/typespec-azure/pull/987, mainly for 1 and 2 format. Format 3 will be implemented in another PR.

Azure / typespec-azure

[Multipart] Support model format for `@multipartBody` #1090