API support for historical data sets

[iamleeg]

Is your feature request related to a problem? Please describe.

If someone publishes a paper based on analysis of global.health data gathered at a particular date, we need other people to be able to retrieve the same data to reproduce the analysis. As it stands you can only get whatever's current.

Describe the solution you'd like

From a client perspective this is easy: API requests to fetch data need to have a date parameter so that you can ask for the (perhaps filtered) data as it appeared at a particular time. How we implement this I don't yet know. We have versioned snapshots of all of the cases, so we can reconstruct the data set as it appeared by limiting ourselves to documents created before the requested date and using the version history to roll back subsequent updates. I don't know how slow that will make it?

Describe alternatives you've considered

Additional context