Add native event support for the vega_card

It would be a nice feature if events could also be received from the altair plots, as it is currently the case with bokeh plots (https://wave.h2o.ai/docs/examples/plot-bokeh-callbacks). As discussed on discord, it seems that vega can render altair plots (https://altair-viz.github.io/user_guide/internals.html), so a native event support could be added for the existing vega card.

Start with the existing vega card and compare its event handling with the bokeh plot callback example linked in the issue. Determine how Altair plots are rendered through Vega and define what event payload the card should expose; done means Altair-generated plots can deliver native events through the vega card.

Written by the indexing model from the issue text.