Format dashboards exported with Kibana

When a dashboard is exported with Kibana and stored as a package, its JSON embedded objects are not decoded, but they are when exported using `elastic-package export`.

When running `elastic-package format`, dashboards exported by Kibana are formatted as any other JSON file, but its embedded objects are not decoded.

For consistency of both methods of exporting dashboards, it'd be nice if `elastic-package format` would decode embedded objects in Kibana dashboards when they were exported directly from Kibana.