envoy: Never use x-forwarded-for header, add for Cilium Ingress

[jrajahalme]

Envoy by default gets the source address from the x-forwarded-for header, if present. Always add an explicit use_remote_address: true for Envoy HTTP Connection Manager configuration to disable the default behavior.

Also add skip_xff_append: true config to keep the existing behavior of not adding x-forwarded-for headers, except for Cilium Ingress, where we now use skip_xff_append: false to explicitly append the source IP to x-forwarded-for header so that Hubble flow records have a trace for the original source of the traffic traversing Cilium Ingress.

Fixes: #25630

Fix incorrect hubble flow data when HTTP requests contain an `x-forwarded-for` header by adding an explicit `use_remote_address: true` config to Envoy HTTP configuration to always use the actual remote address of the incoming connection rather than the value of `x-forwarded-for` header, which may originate from an untrusted source. This change has no effect on Cilium policy enforcement where the source security identity is always resolved before HTTP headers are parsed. Previous Cilium behavior of not adding `x-forwarded-for` headers is retained via an explicit `skip_xff_append: true` config setting, except for Cilium Ingress where the source IP address is now appended to `x-forwarded-for` header.

[jrajahalme]

/test

[sayboras]

LGTM ✅