ability to disable indexing and to enable without admin key

[drzraf]

What did you expect to happen?

I installed the system on a preproduction server.
I expected it to fetch data from production, but not to write to the index

What happened instead?

Production index is rewritten by data from preproduction instance

Technical info

• WordPress version: 4.9.8
• Algolia Search plugin version: 2.11.22.11.2

I would like more control on indexing vs search + production vs preproduction.

1. Algolia provides a search-only API key, but WordPress plugin does not work if both key (search-only + admin) are set. This is the first issue, because I would like preproductions servers to only rely upon the former.

2. Even so, in the event I would like to test the indexing to, I'd like to be able to disable. Currently there is no hook that I know, to disable indexing on an installation (for example to restrict for a given value of WP_ENV/developer/docker vm/hostname/machine/...). Whether from cli or admin UI, there is always a re_index function calling re_index method. A filter to disable this behavior (CLI | UI) would be welcome.