Update /node/health endpoint to ease integration with probe systems

[Magicking]

Problem
Somes probe systems uses HTTP status code as yes(200-2XX)/no(503) rather than yes(200)/maybe(206)/no(503), requiring complex configuration for integration (e.g: Kubernetes probes, shell scripts, ...).

To help those systems a non-breaking update of the API spec can ease the integration, adding an optional integer query parameter syncing_status to the getHealth endpoint, modifying the return status code of the maybe from 206 to the value of syncing_status.

A proposition of this update is currently implemented in Teku.

Actual behavior:
An endpoint answering:

• 200 when fully functional and synced
• 206 when syncing
• 503 when the service is not functional

Desired behavior:
An endpoint behaving this way:

• 200 when fully functional
• syncing_status (e.g: 200, 206(default), 503 or XXX) when syncing
• 503 when the service is not fully functional