Improve robustness of reporting scripts

[fhoering]

As a follow up of the discussion in #1354 several limitations of reporting scripts came up.

They are summarized in the documentation:
https://github.com/WICG/turtledove/blob/main/PA_implementation_overview.md#phase-3---reporting-scripts

In particular reporting scripts compete for resources with bidding scripts and might be lost when the running auction frame is navigated away.

Reporting is one of the most important features as it is used for billing. So any robustness improvement made here would be useful. It should be noted that it might not be necessary to send out reports in strict real time.
Some delay of sending out those events could be acceptable if it improves robustness and improves resource availability for bidding and scoring.