PoC: add cargo as a license data source

Note: This issue has been superseded by Add Cargo support to License Scanning (&13093 - closed) • Unassigned (see license-db section).

Problem to solve

Update license-db to add a new source of licenses for cargo packages.

Proposal

1. Update license-feeder to fetch latest snapshot from bigquery dataset and compare it to last snapshot processed.
2. Add a new interfacer type for cargo.
   1. Add topic and subscription for cargo.
   2. Uses bigquery to fetch data newer than the snapshot supplied.
   3. Convert depsdev licenses to names used internally.
   4. Publish package data to the processor.

Verification Steps

• Contrast ease of access (i.e. replicating couchdb vs bigquery for npm or accessing registries directly)