update helm chart to use preferred backends for gke

[rlakhtakia]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Enable helm chart to deploy multiple EPP services and GCPBackendPolicies for gke preferred backends.

Release note (write NONE if no user-facing change):

Enable helm chart to deploy epp with GKE preferred backends

Test Results

Active-Passive HA (preferredReplicas: 1, defaultReplicas: 1)

Test Scenario	Traffic Profile	Succeeded / Total	Median Latency (p50)	Notes
Baseline Routing	20 sequential requests	20 / 20 (0 dropped)	0.092 s	Normal routing to active Preferred Pod 0.
Live Failover	Continuous stream during scale deploy ... --replicas=0	24 / 25 (1 timed out)	0.092 s	1 timeout (HTTP 504) during ~3s SIGTERM window; 100% instant failover to warm standby pod once marked unhealthy.
Graceful Failback	Continuous stream during scale up to 1 replica	25 / 25 (0 dropped)	0.092 s	0 dropped connections. Traffic continues hitting standby pod until GCP probers mark recovered primary pod healthy.
Load Spillover	100 concurrent requests (~40 req/s vs maxRate: 5)	99 / 100 (1 dropped)	0.100 s	Excess burst traffic spills over natively in Envoy memory to standby tier without dropped connections.

Active-Active HA (preferredReplicas: 2, defaultReplicas: 2)

Test Scenario	Traffic Profile	Succeeded / Total	Median Latency (p50)	Notes
Active-Active Baseline	50 concurrent requests across 10 threads	50 / 50 (0 dropped)	0.104 s	Traffic actively balances across both leader replicas (...-6lg2r and ...-ngm7w).
Scenario A: 1 Active Pod Down	Stream during scale deploy ... --replicas=1	25 / 25 (0 dropped)	0.104 s	When 1 active leader pod crashes or scales down, the remaining active pod takes over 100% of Preferred traffic without dropping connections.
Scenario B: Both Actives Down	Stream during scale deploy ... --replicas=0	25 / 25 (0 dropped)	0.100 s	Once the last active Preferred leader pod terminates, 100% of incoming traffic shifts seamlessly to the warm standby backup pod.

[liu-cong]

can you add a comment on what preferred and default replicas mean? And what does it mean to set a >1 value

[liu-cong]

Shall we update the HealthCheckPolicy to also loop over the additional backup services?