Add isolation levels to SQL Server Connector

[mikedodd]

Extending this concept to the SqlServer Connector

• Configuration in database.php for "Transaction isolation level" ideas#2228
• bdf7699

we currently ran into the situation where we need to set the transaction isolation level for SqlServer.

We implemented it and thought maybe it makes sense to bring that into laravel core, as laravel currently lacks that possibility, whereas Doctrine provides that possibility.

We currently configure it in database.php

return [
    'connections' => [
          'default' => [
                ...
               'host' => env('DB_HOST'),
               ...
               'isolation_level' => env('DB_ISOLATION_LEVEL', 'REPEATABLE READ'),
              ....
            ]
      ]
];

This ha been added already for the MySql connector and so I am following the same concept for the MsSQL connector.