[7.x] Apply model connection name to Database validation rules

[stevebauman]

This change applies the connection name (if it is not null) to database validation rules, where a Model class is given.

This really helps when working with multiple databases in the same application and having to create rules on models that do not use the default application connection.

Without this change:

use App\Sqlsrv\Transaction;
use Illuminate\Validation\Rule;

$model = new Transaction;

$rule = implode('.', [$model->getConnectionName(), $model->getTable()]);

Rule::exists($rule, 'id');

With this change:

use App\Sqlsrv\Transaction;
use Illuminate\Validation\Rule;

Rule::exists(Transaction::class, 'id');

If the connection name is null on the model, the connection name is not applied. There is no BC break with this change.

Please let me know if anything needs to be modified for styling rules. Thanks!