Implement tsp namespace for http-client-csharp

[ArcturusZhang]

Fixes #5442
Fixes #5471
Fixes #5563
Fixes Azure/azure-sdk-for-net#47670

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. Azure generator still needs this, we will add it back in azure emitter (temporary until azure generator decides how to deal with it)
3. removes the use-model-namespace configuration from the emitter because we no longer need it. Azure generator still needs this, we will add it back in azure emitter (temporary until azure generator decides how to deal with 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.

We also have a piece of logic that automatically renames the client Models to ModelsOps in our emitter. We should not have it in the emitter any way. This part is removed. Azure part needs it, therefore we will add the same logic back in azure generator.

Update:
I have to add the configuration namespace back because autorest.csharp is still strongly coupled with this configuration, removing it causes massive changes on that side. Therefore I decided that we keep it here for now, we still write its value to configuration.json for backward compatibility, but MGC will not read it nor use it.

[azure-sdk]

API change check

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

Microsoft.Generator.CSharp.Input
@typespec/http-client-csharp
Microsoft.Generator.CSharp
Microsoft.Generator.CSharp.ClientModel

[JoshLove-msft]

Based on the discussion in the DPG Redmond Sync channel, it is sounding like we will need to support the namespace override from tspconfig.yaml.

[JoshLove-msft]

Based on the discussion in the DPG Redmond Sync channel, it is sounding like we will need to support the namespace override from tspconfig.yaml.

Nevermind - the tspconfig override only needs to be supported in the Azure plugin - not for unbranded.

[JoshLove-msft]

Please file the follow up issue to remove namespace from MGC emitter options along with the steps that would need to be taken in autorest.csharp to upgrade. Ideally, we could just remove it and then leave autorest.csharp pinned to an earlier version.