Create two new events in the remote lib to allow loading custom plugin fields.

With a custom plugin, you can manipulate the form fields using events like "mail:mail_domain:on_after_formdef" and "mail:mail_domain:on_remote_after_formdef", but the remote API doesn't trigger either of these events. With this commit, we now have two new events that allow us to make modifications like inserting custom fields.

I think that having two new events for remote calls is clearer since in these, for example, the "tabs" are replaced with "fields" and it's easier to identify the available data.