Wait for privacy sandbox attestations loaded before Attribution
Reporting processing at start up

Attribution Reporting processing checks privacy sandbox attestations,
and will be rejected if the attestations are not loaded yet.

This CL introduces an observer for privacy sandbox attestations, which
will be notified when the attestations are loaded. Attribution
Reporting will enqueue events in memory until it's notified that the
attestations are loaded. Similarly offline reports are not attempted
to be sent until then. A timeout of 5 minutes is added in case that
it's taking too long.

Bug: 1498937
Change-Id: I69fc33c9a73a96ab6519fe2df633613b3570c90a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5001408
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Xiaochen Zhou <xiaochenzh@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1219537}