Updates storage to persist attribution scope data

https://github.com/WICG/attribution-reporting-api/pull/1365

The migration in this change is light as the column is null-able.
Although null-able columns are not recommended in the SQL guidance, we
feel it's appropriate here as we are not performing any complex
conditional queries with it and it better represents its nullable
counterpart in C++.

Bug: 344081621
Change-Id: I1e56202839e9a3754a9e49709158556f06927a14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5600507
Reviewed-by: John Delaney <johnidel@chromium.org>
Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Thomas Quintanilla <tquintanilla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1346089}