Support for standalone index fields and query conditions on this fields

[Namoshek]

This PR adds support for standalone fields in the database which can be filtered on using the Builder::where($column, $value) method of the search builder.

To use this new feature, use the following steps:

1. Add a migration for an additional column on the scout_index table:

return new class extends Migration {
    public function up(): void
    {
        Schema::table('scout_index', function (Blueprint $table) {
            $table->uuid('tenant_id');
        });
    }
    public function down(): void
    {
        Schema::table('scout_index', function (Blueprint $table) {
            $table->dropColumn(['tenant_id']);
        });
    }
};

2. Index data for the new column by wrapping the value in StandaloneField:

class User extends Model
{
    public function toSearchableArray(): array
    {
        return [
            'id' => $this->id,
            'name' => $this->name,
            'tenant_id' => new StandaloneField($this->tenant_id),
        ];
    }
}

3. Filter on the additional column using Builder::where():

User::search('Max Mustermann')
    ->where('tenant_id', $tenantId)
    ->get();

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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1.1% Duplication