[5.6] Support multiple addresses in array

[ghost]

#23760 did a great job that added support multiple cc/bcc recipient for MailMessage.
But in most of case we don't call cc/bcc method for every single recipient, instead we want pass a array of addresses into it.

<?php

$receivers = User::query()->where('is_bcc_receiver', true)->all()->pluck('email', 'name');

// The origin way:
$message = new MailMessage;
foreach ($receivers as $name => $email) {
  $message->bcc($email, $name);
}

// This PR support the following way:
$message = (new MailMessage)
           ->bcc($receivers);

[GrahamCampbell]

Missing type in phpdoc.

[GrahamCampbell]

Laravel doesn't use return type syntax like this.

[ghost]

@GrahamCampbell fixed.