Add an API to get stale dashboards and cards (#44711)

* Make an internal API for retrieving stale content

This introduces the `metabase.stale` module, which has one public
function (so far): `metabase.stale/find-candidates`. This takes a set of
`collection-ids`, a cutoff date, and pagination/sort info.

Next, we introduce an HTTP API endpoint that will figure out the appropriate
set of `collection-ids` for a given user/request, call this function,
and return the results in the shape we want.

Also: for e2e tests, we need a way to mark a card or dashboard as stale. I
think the easiest way to do this is to just create a testing endpoint
that we can call to update the `last_used_at` or `last_viewed_at` of the
card/dashboard to set it to ~7 months ago.


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Co-authored-by: bryan <bryan.maass@gmail.com>