Apply decorators to the inner type of a `HttpPart` property.

### Clear and concise description of the problem

It would be really convenient to be able to apply decorators to the fields of a multipart model without having to extract the type into a named type.

For example:

```tsp
model Foo {
@maxItems(4)
bar: HttpPart;
}
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.