[typespec-azure-core] Add new model `MultiPartFile` with required metadata

typespec/http has native model [`File`](https://github.com/microsoft/typespec/blob/2eac90cd1e33c39d9e3e32e00d00920f7f1d606a/packages/http/lib/http.tsp#L110-L114) with optional metadata `contentType` and `filename` while many azure services (e.g. Face/openai) all ask filename as required. Currently typespec author has to define the model by themselves like:
```
model FileRequiredMeta extends File {
filename: string
}
```
After discussion, most devs pretend to add the model with required metadata to make it more convenient for typespec author. What's more, SDK could add new Type in core library for this type.