[Feature Request] Support Azure DevOps Extension create/package

**Is your feature request related to a problem? Please describe.**
Currently, I need to leverage both `az devops` and `tfx extension` to complete my workflow for managing extensions and customizing Azure DevOps.

It would be great if `az devops` could take care of this functionality as well since there is already a heavy amount of overlap between these tools for extension management.

**Describe the solution you'd like**
It would be great if `az devops` could support the follow commands already in `tfx extension`:

```
tfx extension [command]
- create: Create a vsix package for an extension.
- init: Initialize a directory for development of a new Azure DevOps extension.
- isvalid: Show the validation status of a given extension.
- share: Share an Azure DevOps Extension with Azure DevOps Organizations.
- unpublish: Unpublish (delete) an extension from the Marketplace.
- unshare: Unshare a Azure Devops Extension with Azure DevOps Organizations.
- publish: Publish a Visual Studio Marketplace Extension.
```