Add Sandbox Mode onto SendGridMessage

[zbrody]

This package served our needs perfectly but I noticed while using the package in my project, we were struggling to enable sandbox mode to ensure emails are not sent to the recipients.

This PR exposes a new 'enableSandboxMode' function on the SendGridMessage class.

We think this could be a nice feature to include in the package as I think other developers can make use of it by chaining on a new function within the SendGridMessage which allows the user to have sandbox enabled.

The default behaviour of the package remains unchanged and the tests have been updated

public function toSendGrid($notifiable): SendGridMessage
{
    return (new SendGridMessage(config('services.sendgrid.templates.default')))
        ->enableSandboxMode(config('services.sendgrid.sandbox_enabled'))
        ->payload([
            ...
        ]);
}

[aozisik]

Thank you very much for your contribution @zbrody! I'll revise a few things and make a new release soon.