Implement Admin API

[MrOrz]

Design doc: https://g0v.hackmd.io/um7DyY_ESbu2LL78kLw3pg?view#API

• Implement feTS OpenAPI server under src/adm
  • OpenAPI spec definition (input & output) is in src/admin/index
  • Utility function goes to src/adm/util
  • Handler logic goes to src/adm/handlers
• Check Cloudflare token on production (and staging) environments https://developers.cloudflare.com/cloudflare-one/identity/authorization-cookie/validating-json/#javascript-example
• Auth plugin that extracts user information and reject non-Cloudflare connections (on staging & production)
• Audit log for all requests on production (and staging) environments. Data include:
  • user (email or service token's client ID)
  • command (endpoint URL)
  • command args (request)
  • response

[coveralls]

coverage: 88.312%. remained the same when pulling 8aa0a79d87f3531ff061001f1177723de1c60a4d on pubsub-cmd into 0179daaf347673ff3205779d7012ee37a2a47cc1 on master.

[coveralls]

coverage: 83.371%. remained the same when pulling d4407a1fccbfefc10272d1b004d4b08838bf6f84 on pubsub-cmd into a43812f20b43daaa92a27bc42dcbc30912536230 on master.