feat: add algolia_clear_index_if_existing filter

[sebastianthulin]

A part of bringing multisite support to algolia wordpress: Allows a developer to turn of automatic index truncating. This is required to prevent a site from erasing other sites data.

Previously discussed here: https://github.com/algolia/algoliasearch-wordpress/pull/745

[sebastianthulin]

Hi! Is it possible to get this merged until next week? I would love to see that, to avoid to make a new fork for my specific use.

[MWDelaney]

I have spent my entire morning basically looking for this feature so that I could combine multiple sites into a single index.

For reference by anyone else finding this (and for some SEO value) this fix lets you combine results from multiple WordPress sites into a single Algolia index. Again, for Google, this can allow cross-site, or multiple site search for Algolia and WordPress.

Referenced from the previous PR, too, here's the filter you'd use to disable clearing of the index:

add_filter('algolia_clear_index_if_existing', function ($state) {
    return false;
});

[sebastianthulin]

@rayrutjes Any progress on this?

[rayrutjes]

Thanks a lot for your active participation in this plugin. We wanted to let you know that we chose to go in another direction regarding the way we integrate Algolia to WordPress. More information can be found here.