Facilitate use of ARM templates in bundles

[itowlson]

Proposal is to be able to browse to a live resource in Azure and extract its ARM template to a CNAB component. Though the template download is likely to be a feature of the Azure extension rather than the Duffle extension. What we can do though is provide a convenient way to expose ARM template parameters as CNAB bundle parameters. (These are defined for bundle.json but not for duffle.toml...)

[itowlson]

The Azure Resource Manager Tools extension provides for the editing of ARM templates, but not (as far as I can tell) for extracting them from live resources.