Allow the Explain panel to be populated by Javascript in a Custom API endpoint

[flaxsearch]

Is your feature request related to a problem? Please describe.
We can return scoring information from some search APIs, but it would be great if we can surface this in the Explain panel to the right of each result.

Describe the solution you'd like
Add another method to the Endpoint mapper e.g. scoreMapper to allow any block of JSON to be shown on the right.

As Eric suggested:

[apavlychev]

We have a problem like this with dynamic queries via a custom API with a elasticSearch. It will be nice to add a endpoint for a explain panel or for elasticSearch of query in every request.