Support for Microsoft.Graph.Beta sdk

[baywet]

As the Microsoft Graph SDK team release support for the Beta version of the Graph with the dotnet SDK it'd be nice to have a beta version of this extension as well under preview to match the behavior.
It's probably only a matter of having two project files and/or nuspec files and it'd allow people to use this extension in combination with the beta endpoint without having to craft requests manually.
Today, because the beta sdk is on the same namespace as the v1.0 one, if you use both the beta sdk and this extension, you end up having conflicts for every entity.
https://www.nuget.org/packages/Microsoft.Graph.Beta/