Add gethealthinfo RPC call and /health HTTP endpoint

[dmidem]

This PR introduces a new health check feature for Zebra:

• Adds a gethealthinfo JSON-RPC method that returns a liveness/readiness status and build metadata.
• Makes the same data available via a simple GET /health HTTP request, using ServerBuilder::health_api.
• The implementation is guarded by the new gethealthinfo-rpc feature flag, so it can be enabled or disabled at build time.
• Includes tests that verify the RPC call and the HTTP endpoint (when the feature is enabled).

[dmidem]

Clossed in favour of #82