[Tracking] The SDK should not crash if there is a new derived type added to the metadata

[maisarissi]

SDK should not crash if there is a new derived type added in the metadata, otherwise older apps built with the SDK would crash when there is a new type in the API.
SDK should treat abstract types as concrete and create an instance of the base type not only for Entities as well as for complex types.

Per initial investigation, we don't emit abstract for Entities.

<EntityType Name="conversationMember" BaseType="graph.entity" Abstract="true">
<Property Name="displayName" Type="Edm.String"/>
<Property Name="roles" Type="Collection(Edm.String)"/>
<Property Name="visibleHistoryStartDateTime" Type="Edm.DateTimeOffset"/>
</EntityType>

[JsonConverter(typeof(DerivedTypeConverter<ConversationMember>))]
public partial class ConversationMember : Entity
{

Users cannot create instances of these classes.

///<summary>
/// The internal ConversationMember constructor
///</summary>
protected internal ConversationMember()
{
// Don't allow initialization of abstract entity types
}

But it seems like we emit abstract for complex types.

[JsonConverter(typeof(DerivedTypeConverter<EducationResource>))]
public abstract partial class EducationResource
{

FYI @darrelmiller @garethj-msft