GraphQL EpicsResolver does not handle `timeframe` arguments

The startDate and endDate arguments are marked as deprecated in favor of timeframe: { start: xxx, end: yyy}.

But this new format does not work for epics.

We pass the args directly to EpicsFinder and that uses TimeFrameFilter. But TimeFrameFilter does not handle the timeframe parameter.

Initially reported in !15110 (comment 591054783)