[Instantsearch] Scout indexing products & categories

[Jade-GG]

Requires this patch for the products to index properly. Without it, only the first 500 products get indexed.

This PR introduces a generic version of the Searchable trait that we can put onto any Eloquent model to make it indexable by Scout, as well as some generic functions that can be hooked into to filter/query the data to your liking. This also replaces the indexer commands with one generic IndexCommand.

A few notes:

• The base Rapidez models will automatically get detected as a Scout model---any others need to be added into the indexer.php config file under extra_models. Is this the best way to go?
• I've kept function names as is, however it might be useful to change them. For example, makeAllSearchableUsing() might be better if we call a new function inside it called something like searchableQuery()
• We don't have some sort of initialization function before indexing, so I've put the maxPositions and categories variables for the Product model into Cache with the array driver. There may be a cleaner way to do this.
• I didn't touch any of the actual code that was already inside of the IndexProductsCommand for now to make sure we keep feature parity, but the withCategories function can probably be refactored?
• The Category model had a weird custom primary key name set which broke Scout. I couldn't find anything that this was actually used for, so I restored it back to what it should be.
• I've put the index prefix inside of the ConfigComposer's getConfig() function. I'm not sure if this is the best way to do this.

[indykoning]

Is it possible to infer this from the model itself? It would save us from setting this value which thanks to your changes doesn't seem needed anymore.

Perhaps a function that reads from an empty model variable/constant, which if it's not set Str::snake() on the class name

[royduin]

That's also an option; currently we do this differently on multiple places, for example:

• The category index is defined here: https://github.com/rapidez/core/pull/735/files#diff-694b39ded25e81aa9cfa9de2b404533c0f924a4919b0f68b38aebfb0741ff68fR1 which is using this config: https://github.com/rapidez/core/pull/735/files#diff-fe38fec1955243f9e20c1cdc08b64167096d11ee224c3c86fc8e4f4671750a51R64
• For the products we've this: https://github.com/rapidez/core/pull/706/files#diff-b30cee12bf5e6816f22522e4ab112a4dc162789811fdcd04c7df321dcb438dbbR132 which can be used in the frontend with config.index

[Jade-GG]

Is it possible to infer this from the model itself? It would save us from setting this value which thanks to your changes doesn't seem needed anymore.

Perhaps a function that reads from an empty model variable/constant, which if it's not set Str::snake() on the class name

I had done this originally, but figured we might want to keep flexibility up for now, and it might not work properly if you overwrite a model with a different class name (e.g. if your code standards require you to call your extended version of a product model ProductExt or whatever).

There's also the thing where it checks if you tried to use Scout directly. Although we'd probably be better off using a different solution for that eventually.