Design how we should write in Enum Expressions for serialization and deserialization

[ArcturusZhang]

Since we now make the serialization extendable by plugins, and the EnumExpression has two expressions of serializing/deserializing the enum which involves the serialization/deserialization.

Previously in autorest.csharp, we never consider this to be extendable by 3rd plugin therefore it is safe that we could let it to call our method, but according to our current design, the serialization could have multiple classes, we could never know which method in which class to use.

Therefore hereby we open this issue to track.