Add ProviderNamespace property to PSResource

## Description of the new feature
Add a property to `PSResource` (as returned from `Get-AzResource`, etc.) called `ProviderNamespace`. Then we can pipe resources into the `*-AzResourceProvider` cmdlets.

This seems like a much less error prone and brittle than encoding knowledge of the [ResourceType format](https://github.com/MicrosoftDocs/azure-docs/issues/17996#issuecomment-541102990) into every script.

## Proposed implementation details (optional)

Something to the effect the following, although I'm sure this isn't how you'd actually do it:

public class PSResource
{
public string ProviderNamespace => this.ResourceType?.Split(new [] {'/'}, 2)?.FirstOrDefault();
}