Specify the ooni-backend HTTP API for defining and exposing collection policy

[aagbsn]

Determine what should be specified -- input sets, specific tests, test decks?
Determine a syntax for how policy is specified.
Determine where (on which system/component/file) policy will be specified.
Specify any additional state between ooni-probe and ooni-backend.

The API should expose the inputs that are supported by the collector backend and the list of test decks that are curated by the collector

See also:
https://github.com/TheTorProject/ooni-probe/issues/109
https://github.com/TheTorProject/ooni-probe/issues/113
https://github.com/TheTorProject/ooni-probe/issues/114

[hellais]

This is now done.