Adds the ability to attach files

[timstl]

Adds the ability to attach files to a SendGrid email (https://github.com/sendgrid/sendgrid-php/blob/main/USE_CASES.md#attachments) with new attach() function.

attach() accepts an array of attachments, allowing for multiple files. Each attachment array should contain the content, type, filename, and (optionally) the disposition.

The file content should be base64 encoded prior to attaching. I found that without doing this, the queued jobs were unable to json_encode the payload and failed.

Example:

// At this point $content has been created using file_get_contents, by generating a PDF, or another means.
// Pass this $attachments array into a Notification class with the payload, templateId, etc.
$attachments = array(
    array(
        'content' => base64_encode($content),
        'type' => 'application/pdf',
        'filename' => 'filename.pdf'
    )
);


// In Notification class, pass them to SendGridMessage's attach function.
public function toSendGrid($notifiable)
{
    $email = ( new \NotificationChannels\SendGrid\SendGridMessage($this->templateId) );
    $email->payload($this->payload);

    if ($this->attachments && !empty($this->attachments)) {
        $email->attach($this->attachments);
    }

    return $email;
}

[aozisik]

Thank you for the contribution @timstl. Attachments is a nice addition! I'm seeing some potential improvements like supporting different ways of attaching / embedding files. I'll merge this to dev and work on it a bit as well.