[attribution_reporting] Add L1 privacy budget for aggregate reports

https://github.com/WICG/conversion-measurement-api/blob/main/AGGREGATE.md#privacy-budgeting

We add two new columns, bucket and value, to the existing rate_limits
table, the latter of which contains the contributions from the aggregate
report. This allows the limit-calculation code to be shared between
event-level and aggregate reports: for the former, we specify 1 as the
value.

We also take this DB migration opportunity to make the
rate_limits.rate_limit_id column NOT NULL.

Change-Id: I3ab15faacd1004d1502c36a1d2222c09d964da79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3085887


Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915941}