Amazon SES Mail Driver

As per the proposal in #5349 here is an implementation of AWS SES as a mail transport driver.

It requires three configuration keys places in the `services.php` file for SES; `key`, `secret` and `region`, similar to SQS queues configuration.

I'm not sure if the instantiation of the `SesClient` should be moved from the `MailServiceProvider` to somewhere else - the `SqsClient` for queues is instantiated in a queue connector, but mail transports don't have connectors.