[10.x] Provide searchable data array with primary key and value for MeiliSearch

[mmachatschek]

Since getScoutKeyName() and getScoutKey() return the expected data (#509) we can now build the searchable data ourselves.

This adds consistancy to the already implemented collection engine and Algolia engine that make use of a "predefined" primary key.

Furthermore, this reduces bugs on the implementation side for users that override the toSearchableArray() method and forget to add the primary key.

[taylorotwell]

Any breaking changes for applications already using MeiliSearch and Scout 9?

[mmachatschek]

I don't consider this as breaking, because people customizing the toSearchableArray() already need to provide the primary key and value with which the data gets fetched from the database later. The array_merge() will handle duplicate data in the payload that gets sent to MeiliSearch.

I created #547 that fixes it on the 9.x branch with the "incorrect" use of scout key name handling.