renderUrl property

Experimental field that can be used during the [FLEDGE Origin Trial](/authorized-buyers/rtb/fledge-origin-trial).

The URL to fetch an interest group ad used in [TURTLEDOVE on-device auction](https://github.com/WICG/turtledove/blob/main/FLEDGE.md#1-browsers-record-interest-groups"). This should be unique among all creatives for a given accountId. This URL should be the same as the URL returned by [generateBid()](https://github.com/WICG/turtledove/blob/main/FLEDGE.md#32-on-device-bidding).