[DISCUSS] Expose Post-processing API and Unify Chart Controls

[Jesse Yang]

Hi All,

I've synthesized my proposal to expose the post-processing API as a SIP and 
expanded the scope a little bit.

Please discuss & subscribe here: 
https://github.com/apache/incubator-superset/issues/9887 
<https://github.com/apache/incubator-superset/issues/9887>


Thank you,

Jesse Yang