[11.x] Make MultipleInstanceManager driver field customizable

[princejohnsantillan]

It's not always the case that the field is driver. Mail for example uses transport.

I want to create my own SMS Manager and my config looks like this:

return [
  'default' => env('SMS_CHANNEL', 'vonage'),
  
  'channels' => [
    
    'vonage' => [
      'provider' => 'vonage',
      'key' => env('SMS_VONAGE_KEY'),
      'secret' => env('SMS_VONAGE_SECRET'),
    ],
    
    'twilio' => [
      'provider' => 'twilio',
      'account_sid' => env('SMS_TWILIO_ACCOUNT_SID'),
      'auth_token' => env('SMS_TWILIO_AUTH_TOKEN'),
    ],
    
    'log' => [
      'provider' => 'log',
    ]
  ]
]

I use provider here but the abstract class currently forces us to use driver only.