[Mailer][Mailjet] MailjetApiTransport Sandbox Mode

### Description

Mailjet introduced a Sandbox Mode https://dev.mailjet.com/email/guides/send-api-v31/#sandbox-mode that allows to use the API but not sending any email e.g. for debugging. 

https://github.com/symfony/mailjet-mailer/compare/5.4...mdawart:mailjet-mailer:sandbox

### Example

`MAILER_DSN=mailjet+api://$PUBLIC_KEY:$PRIVATE_KEY@default?sandbox=true`