Support multiple namespaces in client library code generation

[srnagar]

Clear and concise description of the problem

Currently, code generators for .NET, Java and Python only support generating client library in a single namespace. Generators use the namespace defined in tspconfig.yaml and ignore the namespaces defined in TypeSpec. This works when we only have a single namespace.

However, there are cases where we need multiple namespaces in the generated library to allow different clients to be in different namespaces (for e.g. OpenAI and Search). So, the proposal is for generators to use the namespaces defined in TypeSpec as the namespaces in client libraries too and not use tspconfig.yaml.

More details on the proposal can be found here.

Checklist

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• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Spec

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1. Namespace refactoring Scenario tests Azure/cadl-ranch#758
   +0
   
2. Namespace refactoring TCGC Author doc Azure/typespec-azure#1722
   docs lib:tcgc +2
   

Implementation

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1. Namespace refactoring TCGC Implementation Azure/typespec-azure#1723
   lib:tcgc +1
   
2. Namespace refactoring Implementation Azure/autorest.java#2968
   DPG +1
   
3. Namespace refactoring Implementation Azure/autorest.typescript#2877
   HRLC p0 +2
   
4. Namespace refactoring Implementation #5442
   emitter:client:csharp +1
   
5. Namespace refactoring Implementation Azure/azure-sdk-for-net#47670
   Client CodeGen feature-request +3
   
6. Namespace refactoring Implementation Azure/autorest.go#1452
   +0
   
7. https://github.com/Azure/autorest.cpp/issues/404
8. https://github.com/Azure/typespec-rust/issues/130
9. Support multi namespaces Azure/autorest.python#2795
   +0
   
10. [tcgc] namespaces in TCGC Azure/typespec-azure#1604
    lib:tcgc +1
    

[lirenhe]

To clarify, we need to support multiple namespaces also for unbranded scenario and the scope includes c#, Java, Python, JS Modular

[weidongxu-microsoft]

There is models under typespec.rest.resource, e.g. https://github.com/microsoft/typespec/blob/main/packages/rest/lib/resource.tsp#L115-L117

For now, unbranded won't do any default namespace mapping, so they would appear at
https://github.com/allenjzhang/typespec-e2e-demo/blob/main/petstore/clients/java/src/main/java/typespec/rest/resource/PetUpdate.java

Do we need to do a mapping for models from this typespec.rest.resource? <-- Dec. 12, yes, should be in 0.1.5