Inherit `SubResource` and `WritableSubResource` from resourcemanager instead of replace the types

[ArcturusZhang]

To keep the ability of expanding new properties, we could make the model that would be replaced by SubResource or WriteableSubResource inheriting them instead of replacing

Now we have "safe flatten" in mgmt plane which will automatically flatten the properties with a type of "single property" class. But this does not work when the property is a list of a "single property" class, for instance, a property

public IList<WritableSubResource> Subnets {get;}

here the WritableSubResource here is not its actual type, it is replaced by the generator. When a new property is added to the element in swagger, the replacement will no longer apply therefore we will get a breaking change now.
If we could make the replacement in this way: we generator the class without replacing it, but let it inherit from SubResource or WritableSubResource, the SDK will end up in this way:

public IList<SubnetInfo> Subnets {get;}

public partial class SubnetInfo : WritableSubResource {}

[m-nash]

Wouldn't this be a breaking change?

[ArthurMa1978]

Might cause breaking change, revisit if it worth to do it now