AOT must accommodate init and required properties #1919

In migrating one of my apps to MessagePack v3, I hit a roadblock in that the auto-generated formatter doesn't use the object initializer syntax to set properties with init property accessors. As a result, the project no longer compiles.

Since the system already knows to deserialize property values required by the constructor before invoking the constructor, it should be a relatively simple matter to also collect init properties before invoking the constructor, and then to set the properties using object initializer syntax.