Implement an API call request counter module

[mfranciszkiewicz]

What:

• propose a name for the custom counter (main counter) of total number of requests made by all registered users within the runtime
• in Runtime::start function, spawn a background task responsible for:
  • periodically fetch per-user and per-endpoint counter values from the proxy server for all registered users
  • publish the per-user counter via Runtime API to ExeUnit Supervisor (see the GAP for examples)
  • publish the last known counter value on shutdown

Why:

• we need to provide a number of total calls per authenticated user as a custom runtime counter

Follow GAP-8