[9.x] - Collection Engine: add support for non-scalar values

[macbookandrew]

The Algolia and MeiliSearch engines both allow searchable data to be non-scalar values (e.g., an array of ingredients or genres).

The Collection engine, however, expects all values to be scalar, and during search, throws a TypeError if a model’s toSearchableArray() returns any non-scalar key.

TypeError: mb_strtolower(): Argument #1 ($string) must be of type string, array given in /path/to/project/vendor/laravel/framework/src/Illuminate/Support/Str.php:361
Stack trace:
#0 /path/to/project/vendor/laravel/framework/src/Illuminate/Support/Str.php(361): mb_strtolower(Array, 'UTF-8')
// etc.

This PR is a stupid-simple fix that just JSON-encodes non-scalar values on the theory that it will result in a string that Str::contains() will be able to inspect for the search term.

[macbookandrew]

The biggest benefit I see is being able to use the Collection engine in tests that depend on Scout search results (i.e., testing the content of a page rendered by a search controller).