[5.x] Add algolia prefix option

[bastihilger]

In Algolia it is common to have one "Application" with different indexes for different environments. These indexes can be prefixed, like

• "local_pages", "production_pages", "staging_pages"
• "local_products", "production_products", "staging_products"

and so on.

Laravel scout has an easy solution for this by adding a "prefix" option in the scout config file.

My PR adds a similar solution by having an option in the Statamic search config file in the drivers.algolia array.

Closesstatamic/ideas#1202

[jasonvarga]

I believe this other PR is attempting to solve the same problem: #10435

We can leave this open and we will review both and decide the more appropriate option. Thanks!

[bastihilger]

@jasonvarga I did not see that PR. I'd be happy if one of the two got merged ;-)

[jasonvarga]

On the other PR a good point was made that this sort of functionality would not necessarily be needed only by Algolia. #10435 will address it in a driver-agnostic way. We can continue over there.