Extension publish command

[tjprescott]

Create a command to build and/or publish an extension.

azdev extension build - simply builds the extension
azdev extension publish - builds and uploads the extension to storage (and possibly other locations in the future). Optionally update the index.

This way the flow is simply:

1. azdev setup -r azure-cli-extensions
2. Do you extension development
3. azdev extension publish ...
4. Open PR!

[tjprescott]

Requires Azure/azure-cli#8896 be merged.

[tjprescott]

This also will require a new version of Knack.