ChefExtension support in the azure sdk toolkit

Contribution to the Azure SDK Toolkit from Chef - To support ChefExtension cmdlets.
Note - ClogenyTechnologies has a contract with Chef. We have signed the Microsoft CLA. This new PR has been created to accomodate the "rebase" after this repo had been restructured.
Original PR - https://github.com/Azure/azure-sdk-tools/pull/2576
I think this could be handled by more parameter sets.

S1: ClientRb cannot be specified, ChefServerUrl & ValidationClientName optional
S2: ClientRb is mandatory, ChefServerUrl & ValidationClientName optional

Combined with the Windows/Linux switches, there would be four sets in total.