Session Persistence for ExtAuth

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*Title*: *One line description*

Session Persistence for ExtAuth

*Description*:
>Describe the issue.

Hi,
We have three replicas of our extAuth server and would like to implement session persistence (for example, hashing by source IP).
I noticed that sessionPersistence can be set in HTTPROUTE for backend services. However, it does not apply to extAuth specified in SecurityPolicy. I've also tried the backendSettings under GRPCExtAuthService, but it doesn't work neither.

Here is the config of my SecurityPolicy
```
spec:
extAuth:
bodyToExtAuth:
maxRequestBytes: 16384
failOpen: false
grpc:
backendRefs:
- group: ""
kind: Service
name: aaa
namespace: bbb
port: ccc
backendSettings:
loadBalancer:
consistentHash:
tableSize: 65537
type: SourceIP
type: ConsistentHash
```

Is this feature supported? Any suggestions are appreciated. Thank you.

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