Support for HEADER_FIELD session affinity type on GCPBackendPolicy

[mukesh-nayal]

We would require support for HEADER_FIELD session affinity on GCPBackendPolicy CRD as CLIENT_IP isn't suitable for org with Gateways/NAT enabled.

Reference: https://docs.cloud.google.com/kubernetes-engine/docs/how-to/configure-gateway-resources#:~:text=GCPBackendPolicy%20does%20not%20support%20HEADER_FIELD%20or%20HTTP_COOKIE%20options%20for%20session%20affinity