OONI Run v2.1 add API call for checking multiple descriptors at a time

[hellais]

It was requested by @sdsantos that it might be useful to have an API call to batch requests for changed descriptors by passing in an updated_time.

[hellais]

We should discuss better the requirements with the mobile team, if they could provide an example that would be great.

[sdsantos]

If a user has N descriptors (run V2 links) installed, the app needs to do N api calls to /api/v2/oonirun/links/:ID: to know if there's a new update available. Ideally we should have a custom endpoint that would receive a list of run IDs and a timestamp, and would return the list of descriptors with those run IDs updated since that timestamp.