enhancement: support upstream health checks in BackendTrafficPolicy

[kayx23]

Summary

Please triage whether BackendTrafficPolicy should support upstream health check configuration.

Current behavior

APISIX itself supports active and passive upstream health checks, and API7 Ingress Controller already exposes that capability through ApisixUpstream.

However, BackendTrafficPolicy currently exposes upstream scheme, retries, timeouts, load balancing, and host handling only.

As a result, Gateway API users cannot configure health checks for their backends through BackendTrafficPolicy, even though the underlying APISIX capability already exists and is available through ApisixUpstream.

Use cases affected

• configuring active or passive health checks for Gateway API backends
• keeping backend failover and health probe settings in the same Gateway API-oriented configuration flow

Code context

• api/v1alpha1/backendtrafficpolicy_types.go
• api/v2/apisixupstream_types.go