thanos: query-frontend: Cache Intermittent queries (e.g inside Instant Queries)

It’s very common for rulers to query super long time ranges like *_over_time(metric[1w]). The problem is that it’s just an instant query so totally not supported by the existing cortex query frontend.

Let’s make sure we can contribute something that is capable of doing so, potentially while doing https://github.com/cortexproject/cortex/issues/2178 and https://github.com/thanos-io/thanos/issues/2454