Feature: Search Multiple Indices

[kyleboe]

Ref: https://www.algolia.com/doc/rest-api/search/#search-multiple-indices

I'm working on a PR for this right now. Just wanted to open this up for implementation discussion to see if anyone from the WTTJ team had run into any pitfalls that I should be avoiding.

[StephaneRob]

Hi @kyleboe, it would be great to have this feature.
The multi index search could be just an API on Algoliax module (outside of the Algoliax.Indexer) and relies on existing HTTP client.