Add inspector to Vega vis

[nyurik]

When developing a Vega visualization it is paramount to be able to view the underlying data in the different stages of processing by Vega language. Data inspector is a perfect fit, but it is not being supported by Vega at the moment. We should add the following capabilities to the inspect view:

• View the state of the data sources after all transformations have been applied. This is the same data as viewable from the browser debug console with VEGA_DEBUG.view.data('<source name>'). The standalone Vega editor already implements it in this way:
  
• View all ES requests made by Vega (stats/request/response). This is probably a bit less critical than the data viewer, but highly useful nevertheless.
• We may want to support other internal Vega state such as signal values.

See Vega debugging API.

[elasticmachine]

Pinging @elastic/kibana-app

[monfera]

Assigned after a convo with @timroes

[alexwizp]

hey @monfera , if you don't mind I also would like to work on that.

[timroes]

I'll add to this, the most useful things we currently most likely want in the custom inspector panel:

• View the compiled Vega specification
• View the data tables
• View other information (e.g. signals, like in the editor), the debugging API documentation has a couple of useful explanations in there

[nimocks]

Would also be nice to see the elastic data API calls/responses in this inspector UI on top of what Vega debug can offer.