feat: Reference spec source in DataFlow nodes

[danmarshall]

This PR adds a new property src to DataFlow nodes. This references the specification property which spawned each node. It is helpful to be able to map the scenegraph item to the specification.

Scenario: I have a custom Vega renderer which traverses the scenegraph and renders in WebGL. I am using Vega axes. I want one of these axes rendered in a 3D plane instead of 2D.

Example: In my spec for the axes, I add a new attribute plane:

"axes": [
        { "orient": "bottom", "scale": "x", "title": "X Axis"},
        { "orient": "left", "scale": "y", "title": "Y Axis" },
        { "orient": "left", "scale": "z", "title": "Z Axis", "plane": "z" },
    ],

During the scenegraph render pass, I can get to the mark node generated by the DataFlow, and via the src property I can get to this axis object from the spec and get the plane attribute.

This may also enable other scenarios, such as animation attributes etc.

[jheer]

Thanks for the PR, and apologies for the delayed review. I'm a bit skeptical of annotating the runtime with specification source: this violates a "separation of concerns" across levels of abstraction and will also add bloat to the runtime.

That said, supporting extensibility (e.g., to new renderers) is a nice goal. Vega already supports this to some degree: any arbitrary encoding property can be defined and will be passed through as a property written to scenegraph items. This applies to axes and legends as well, but in those cases requires using a custom encoding block. For example, you could write custom properties to the axis group mark and have your custom renderer respond to those.