Support MergeType in ClientTrafficPolicy

[arkodg]

Description:

Describe the desired behavior, what scenario it enables and how it
would be used.

Add support for merging policies applied at various levels similar to what exists for BTP
https://gateway.envoyproxy.io/latest/concepts/gateway_api_extensions/backend-traffic-policy/#policy-merging

[optional Relevant Links:]

Any extra documentation required to understand the issue.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days.

[funkluk]

Is somebody working on this? As other similar issues are around, do you think it will be solved in the same go? This is quite important for us, along with #6734 .
If not, I'll check if we can put resources on this to implement it.

[arkodg]

cc @kkk777-7

[arkodg]

@funkluk feel free to pick this one up if you can

[daum3ns]

@arkodg i would like to pick this up (after i get #7331 finally merged)

[kkk777-7]

@daum3ns thanks for picking up!
BackendTrafficPolicy's code help you :)
https://github.com/envoyproxy/gateway/blob/main/internal/gatewayapi/backendtrafficpolicy.go