feat: add support for V4 delete contact endpoint

[filfreire]

Hi folks!

There's a case where if folks try to use mailjet-apiv3-go client to delete contacts, e.g. due to GDPR requests, where we get a 401 error.

This is due to the fact that we are sending the DELETE request to /v3 api.

If we use the client's SetURL or SetBaseURL to override the base URL and send it to /v4 instead, the /REST path is still added. This PR adds a workaround to support sending requests like Delete contact to /v4 in a way that won't add the extra /REST path.

Note to the maintainers of mailjet client: this was just a quick "hacky" way/workaround, but I would be happy to help make this PR into a more clean/mergeable state if provided with some guidance.