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
@adamedx , @huangpf 
We have added the default extension version to be the latest Chef version available for that platform.
Do let us know if the PR looks good and can be merged to the master.
Thanks,
Mukta.