Consider implementing `RequestContent<T>` when deriving `Model`

[heaths]

For top-level request models that are for required parameters - and maybe we need an attribute to identify these - we should consider implementing RequestContent<T>. Or maybe we just need a separate derive macro (or attribute macro) for this. It's very boiler plate.

See Azure/typespec-rust#124 (comment) for context

[heaths]

We no longer have a Model trait or derive macro.