[Feature Request]: `/health` RPC endpoint

### Implementation ideas

Right now there is no good way to check that Node RPC is functional/accessible:
- `/` gives HTTP 404
- other endpoints can return app-specific errors

Simple `/health` endpoint enables easy check of RPC availability.
Requirements:
- no cache
- return valid JSON (KISS: `{"status": "ok"}` should be enough for now)
- HTTP status code 200 if everything is fine

In the future, more information may be added to endpoint.

References/reading:
https://testfully.io/blog/api-health-check-monitoring/
https://docs.microsoft.com/en-us/azure/architecture/patterns/health-endpoint-monitoring