Honor Istio DestinationRule session affinity (sticky sessions)

[PetrMc]

kgateway version

2.0

Is your feature request related to a problem? Please describe.

When using kgateway with Istio DestinationRules that have session affinity enabled, the sticky session functionality does not work properly. This is a limitation for users that would like to extend the use of session affinity from their Istio setup without creating additional configurations

As shown in the attached diagram, there are four scenarios:

• In-cluster calls using istio-proxy with sticky sessions: WORKS
• External calls using Istio Ingress Gateway with sticky sessions: WORKS
• In-cluster calls without istio-proxy and sticky sessions: DOES NOT WORK
• External calls with kgateway and sticky sessions: DOES NOT WORK

Describe the solution you'd like

kgateway should properly implement and honor the stickiness settings defined in Istio DestinationRules, similar to how Istio's native Ingress gateway handles them. When a DestinationRule specifies loadBalancer.consistentHash settings for session affinity, 'kgateway` should respect these settings and route subsequent requests from the same client to the same backend pod.

Describe alternatives you've considered

My tests of GW API session affinity, show the following short comings:

• i couldn't find the way for GAWAPI to work passively - it seem to require a token provided by the gatewayto be extracted and added to the subsequent client request to be sticky. that requires customers to change their client app logic. Istio implementation uses client provided header or cookie to maintain session affinity.
• also if istio implementation already has the session affinity configured - it seems logical for kgateway to respect it without requiring user to create more configs

Additional Context

come across an issue that also affects LoadBalancing. #10461