Implement tsp namespace for http-client-csharp

[ArcturusZhang]

Fixes #5442
Fixes #5471

This PR contains the following:

1. now we honor the namespace defined in typespec for both model and clients (short for "namespace is namespace" feature)
2. removes the namespace configuration from the emitter because we no longer need it.
3. removes the model-namespace configuration from the emitter because we no longer need it.
4. reports diagnostic when there are conflicts between namespace segments and subclient names.

Because this change changes the namespace of literally everything, there are quite a few file changed. Majority of them are the generated files changed for the namespace.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

Microsoft.Generator.CSharp.Input