JSON transport API review

[ajduberstein]

As we enter a multi-language ecosystem built on-top of it, the JSON API (see here) would benefit from an API review. Some use cases like the ones below are not effortlessly handled by the current syntax:

• Switching base-map providers (see https://github.com/visgl/deck.gl/pull/4612#discussion_r430784406)
• Interaction handlers (How do we best specify onClick or onHover without writing functions?)
• UI components (can/should the JSON API support UI components like vega-lite or similar frameworks do? Scales and legends, for example)
• Binary transport

[kylebarron]

• Interaction handlers (How do we best specify onClick or onHover without writing functions?)

4637 is somewhat related to this: how to specify values that must be functions