Support /client endpoints

[ans-ioki]

To allow the client to prompt self-updates.

Note that this are the only endpoints besides "auth", that don't need tokens.

Examples:

curl -i -X GET \
  -H "X-Api-Version: 20210101" \
  -H "X-Client-Identifier: dev.platformapi.test.backend" \
  -H "X-Client-Secret: <SECRET>" \
  -H "X-Client-Version: 0.0.1" \
  https://demo.io.ki/api/platform/client

curl -i -X GET \
  -H "X-Api-Version: 1" \
  -H "X-Client-Identifier: dev.passengerapi.test.backend" \
  -H "X-Client-Secret: <SECRET>" \
  -H "X-Client-Version: 0.0.1" \
  https://demo.io.ki/api/passenger/client