Add support for discovery extensions

[SteveL-MSFT]

PR Summary

• Add new subcommand to dsc to list extensions: dsc extension list
• Add new ExtensionManifest which is similar but differs from ResourceManifest. Only discover capability currently supported.
• Enhance existing discovery code to handle both resources and extensions. This includes pre-discoverying extensions when discovering resources since an extension may participate. This included a new enum to handle returning either resources or extensions.
• Write extensions as table when used interactively
• Rename existing functions to generalize for both resource and extension use, this also required making some private functions public

The output from a discovery extension must match:

{ "resourceManifestPath": "<absolute path>" }

PR Context

Fix #681