Implement discriminator of models

[ArcturusZhang]

Fixes #3500

Depending on #3730

Check list:

• base should be abstract, derived should not be abstract
• initialization ctor of base should be protected
• discriminator property should not appear in the initialization ctor of base
• discriminator property should be assigned to its discriminator value in derived type
• base model should have the deserializeProxy attribute
• the serialization of base model should be delegated to the corresponding derived type
• the deserialization of base model should be delegated to the corresponding derived type
• there should be an unknown derived type to handle the unknown values for discriminator value

[azure-sdk]

API change check

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Microsoft.Generator.CSharp