sentry: rateLimit type "whitelist"

[Ivshti]

allow publishers/advertisers to define whitelist of users who can submit events when setting up the campaign; potentially allow dynamic changes

this is still not in the spec, so it still has to be discussed and spec'd

this can also be related to various mechanisms in which you get in the whitelist for some time: provide a (memory bound) PoW solution, complete a captcha and etc.

Example use case: publisher's backend sends impression events rather than frontend (eg opensubtitles subtitle ads)