[Azure Mgmt Generator] Support "specific extension"

[ArcturusZhang]

Now our generator just make resources with "specific extension" as "Extension". This information is not enough.
For instance, for a resource like this:

alias VM = Extension.ExternalResource<>;

resolveArmResources returns a ResolvedResource with a specific "external resource" as its scope, but we did not handle it now, therefore it will have a scope of "Extension".
This could also happen when the resource is written by using a complicated "legacy operation".

This causes an issue in the C# part because C# part actually understands the "Extension" as scope as a resource path like those with scope parameters: /{scope}/providers/...

Therefore we need to refine both parts to make things correct.