[8.x] Add Failover Swift Transport driver

[nerg4l]

Sometimes mailgun is not available in our area. The most recent one in the EU was on Aug 2 for ~2 hours and the one before that was on May 29 for ~5 hours (https://status.mailgun.com/). In each cases our clients might lost users by not getting immediate emails. With a "failover" mail transport we could fallback to a different transport driver in the future and help our clients not to lose users.

Swift_FailoverTransport is already battle tested and I was able to provide backward compatibility for "Laravel <= 6.x style mail configuration files".

This feature was requested in 2014 but it was rejected with a comment "No plans to implement this currently.". #3374

My original plan was to develop a provider (https://github.com/coding-socks/laravel-mail-failover) but the whole logic is so simple that in my opinion this could be added to the core library. If you disagree I can still release my lib on packagist.

[taylorotwell]

Can you explain what it does and how it works?

[nerg4l]

Of course.

Swift_FailoverTransport accepts a list of transports. It always starts using the first transport and if it fails, it will retry the delivery with the next transports until one of them succeeds or all of them fail.

With this feature one can define in their config/mail.php a failover mailer the following way:

    'mailers' => [
        'failover' => [
            'transport' => 'failover',
            'mailers' => [
                'mailgun',
                'postmark',
                'sendmail',
            ],
        ],

        'mailgun' => [
            'transport' => 'mailgun',
        ],

        'postmark' => [
            'transport' => 'postmark',
        ],

        'sendmail' => [
            'transport' => 'sendmail',
            'path' => '/usr/sbin/sendmail -bs',
        ],
    ],

With the above configuration when failover is used Swift_FailoverTransport will attempt to send an email with mailgun mailer. If that fails it will attempt to send an email with postmark mailer. If that fails it will attempt to send an email with sendmail mailer. If that fails it will raise an exception.