Simplifications and fixes

[stratoula]

Summary

This PR:

• simplifies the api as for ESQL queries we don't need the breakdown type. Also we just send a string (the field name) and nothing more. The complexity starts with formulas but we don't have formulas here!
• fixes all the chart types to work properly. At least they work for my examples. I am adding some examples below.

My initial prompt to generate the query is

I want the average of bytes grouped by the top 10 destination. Get the data from the logstash indices

And then I ask a specific chart

Here is a dashboard I created only with AI