Create "eventcallbackurl" lacks grouping

[svoop]

This is actually a shortcoming of the current API (compared to the web UI) and by extension this gem. Since there is not repo for the API itself, I report it here:

Whe adding a callback URL for an event like "clicked", the web UI features a "grouped" checkbox. It assures the callback doesn't fire more than once per second which is very important for efficiency and to prevent a "self-inflicted DoS" of sorts:

Unfortunately, this setting is not available when the callback URL is created using the API:

https://dev.mailjet.com/email/reference/webhook/#v3_post_eventcallbackurl

We're configuring callbacks, webhooks etc programmatically, so this is a serious shortcoming.

Thanks for giving this some though and priority!