[http-client-csharp] Refactor the way how we resolve namespace conflicts in spector test project

Now we have to import every individual csproj of the source code of spector cases, which is very cumbersome.
We might need a more sufficient way to resolve the conflict between classes.

TypeSpec team is planning a feature of aliasing so that when we import another file, we have the ability to change the namespace of everything inside it as an alias. When this is ready, we could update all the spector cases to use it to ensure they never have a conflict between each other.

But this is not ready. We need to do something to achieve it.
1. We could change all those spector cases to ensure they do not have conflicts between each other. Hopefully all those projects with issues come from `azure-spector-spec`
2. We could have a new plugin especially for spector cases, which updates the namespace of those shared problematic types.