[Search API] Before submitting a query, preview the total results for the requested API service

Summary

Viewing a preview of the number of results would be useful for the user who then decides whether or not to launch their analysis.

Steps to reproduce

On a new corpus, fill a query via the search API form.

What is the current bug behavior?

Currently, we only can submit a query. There is no preview of the total of results.

What is the expected correct behavior?

To propose a preview of the total results from each different external database.