Request to add Summary plugin to core plugins

[aszenz]

Perspectives built in plugins are tabular/graph based, for dashboards a card summary is often needed, there is a nice plugin built for this at https://github.com/perspective-community/perspective-summary but it's not actively maintained, it would be great to move it in this repo, would also allow using it with html export since adding third party plugins breaks html export see #2950

[aszenz]

@timkpaine Wdyt about this idea, would you be okay with moving the plugin here