Web API: add an endpoint to list all origins
We want to allow downstream archive users to obtain, and maintain up-to-date, a full list of the origins known by Software Heritage. A reasonable model that we can replicate for this is GitHub repositories API. We can use as page index the origin ID, which is incremental and already "leaked" by the origin search API anyway.
Migrated from T1399 (view on Phabricator)