[Epic] Refinement schema of tspCodeModel.json to align with the types in TCGC (Sdk type part)

[ArcturusZhang]

Our current design in the schema of tspCodeModel.json has quite a few inconsistencies with the models from typespec-client-generator-core (TCGC) and as well as not so reasonable in quite a few places.

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1. Intrinsic types are not meaningful except unknown #4712
   DPG DPG/RLC v2.1 +2
   
2. DateTime and Duration are standalone types in TCGC #4713
   DPG DPG/RLC v2.1 +2
   
3. EnumValueType is just a string instead of an instance of primitive type #4714
   DPG DPG/RLC v2.1 +2
   
4. Emitter needs to adopt TCGC's recent changes on nullable #4701
   DPG DPG/RLC v2.1 +2
   
5. Properties of models in TCGC starts with lower case letter, but ours start with upper case letters #4715
   +0
6. The base type InputType defines the Name property, but not all types have names #4728
   +0
7. Duplicate information in our input type and serialization system #4131
   DPG DPG/RLC v2.1 v3 +3
   
8. Use the usage flag from TCGC directly #4734
   DPG DPG/RLC v2.1 v3 +3
   
9. Remove literalTypeContext in emitter #4746
   DPG v3 +2
10. Consolidate InputModelType with TCGC's SdkModelType #4784
    DPG v3 +2
    
11. Consolidate InputUnionType with TCGC's SdkUnionType #4785
    DPG v3 +2
    
12. Move the flatten handling logic to the generator #4788
    DPG DPG/RLC v2.1 v3 +3
    
13. Consolidate InputListType and InputDictionaryType with TCGC's SdkArrayType and SdkDictionaryType #4789
    DPG v3 +2
14. Consolidate InputModelProperty with TCGC's SdkModelPropertyType #4793
    DPG v3 +2
15. We have a IsEmbeddingsVector in InputListType, TCGC does not #4801
    DPG DPG/RLC v2.1 v3 +3
    
16. Use crossLanguageDefinitionId for type replacement #4868
    DPG v3 +2
    
17. Add decorator fields in every Input types in tspCodeModel.json #4887
    DPG DPG/RLC v2.1 +2