Add index to sp_return_logs to speed up reporting queries

[zachmargolis]

Note: My goal is to deploy this index as a patch release (today?) so that it if goes poorly, we can revert it and clean it up well before the scheduled Thursday deploy

Background:
We're getting consistent timeouts in the new, Combined IAA report see below is one of the queries that's timing out (inside of TotalMonthlyAuthCountsWithinIaaWindow)

The sp_return_logs table is very big, and even using the requested_at index still yields too many rows, so our current hypothesis is that a new index would help to speed things up, assuming the query planner doesn't pick the wrong one.

SELECT
sp_return_logs.user_id
, '202109' AS year_month
, sp_return_logs.ial
, COUNT(sp_return_logs.id) AS auth_count
FROM sp_return_logs
WHERE
sp_return_logs.requested_at BETWEEN '2021-09-01' AND '2021-09-29'
AND sp_return_logs.returned_at IS NOT NULL
AND sp_return_logs.issuer = '...'
GROUP BY
sp_return_logs.user_id
, sp_return_logs.ial