Build a common interface for Policy Processing in Gateway API layer

*Description*:
>Describe the issue.

The Gateway API translator translates multiple policies - EnvoyExtensionPolicies, BackendTrafficPolicies, SecurityPolicies, and BackendTrafficPolicies, that do similar things - validate the policy, translate it and choose the right target and apply the setting to it.

A common policy processor with custom hooks per Policy types could dedup some of the logic to improve maintainability and readibility of the code base

https://github.com/envoyproxy/gateway/blob/main/internal/xds/translator/httpfilters.go is one example

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>Any extra documentation required to understand the issue.