Support for enabling Azure Private Marketplace cli command

**Is your feature request related to a problem? Please describe.**

Unable to enable and manage Private Marketplace in Azure using az CLI

**Describe the solution you'd like**

A subcommand in az cli that will enable private marketplace and allow for management of offers.

`az marketplace set --private true`

`az marketplace list-offers`

`az marketplace add-offer --offer-file offer.json`

**Describe alternatives you've considered**

**Additional context**