Check run link updates in bulk

[sdsantos]

Probe apps check for Run Link updates by fetching the descriptor json through "https://api.ooni.io/api/v2/oonirun/links/ID" and comparing the date updated of the JSON response received (we can't just compare revisions, because some changes do not trigger a new revision number). This is done concurrently and either at app start, or in the background every 24 hours.

Disadvantages:

• Users with a lot of run links installed will do many networks calls that would be unnecessary, since most run links have infrequent updates.
• Once we migrate OONI default test descriptors to Run Links, all users will start having 5 default Links to check for updates, increasing significantly the number of requests to the API.

Proposal

An API endpoint to check for run link updates in bulk.

Request options:

1. Submit a list of run link IDs
2. Submit a list of run link IDs and a timestamp of the last time it checked for updates
3. Submit a list of run link IDs paired with their stored date_updated

Response options:

1. Receive a list of updated run link IDs
2. Receive a list of updated run link JSON payloads