feat: snat azure dns traffic to node ip and remove jump to swift postrouting in iptables legacy

[QxBytes]

Reason for Change:

Changes the ip CNS-added IPTables rules SNAT to from the primary ip to node ip for linux podsubnet scenarios (both azure and cilium cases). CNI-added iptables rules are not modified and windows behavior remains the same (will be modified in a future PR).

Removes the jump to SWIFT-POSTROUTING in iptables-legacy if it is present as CNS is now built with iptables nftables (so we avoid having iptables-legacy and iptables-nftables conflicting with each other). If both rules exist the order of evaluation is nondeterministic.

This change will likely roll out with the change that enables CNS to programSNAT rules in azure cases (previously CNS would only program iptables rules in cilium cases)

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• relevant PR labels added

Notes:
Tested upgrade and downgrade gets back to original state (minus the jump to SWIFT-POSTROUTING in iptables-legacy):
cilium podsubnet case (this case potentially has the jump to SWIFT-POSTROUTING in iptables-legacy as older cns versions programmed iptables-legacy rules)
azure podsubnet case (this scenario should only be using iptables-nftables)
Old SWIFT-POSTROUTING rule is successfully deleted in iptables legacy if it exists

[Copilot]

Pull Request Overview

This PR modifies SNAT behavior for Azure DNS traffic in Linux podsubnet scenarios by changing the source IP from the primary subnet IP to the node IP, and removes conflicting iptables-legacy rules to prevent conflicts with iptables-nftables.

• Changes SNAT target from subnet primary IP to node IP for Azure DNS traffic
• Removes jump to SWIFT-POSTROUTING in iptables-legacy to avoid rule conflicts
• Adds support for iptables-legacy client interface to handle cleanup operations

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cns/restserver/restserver.go	Adds iptablesLegacyClient interface and getter method
cns/restserver/internalapi_windows.go	Implements unsupported legacy iptables for Windows
cns/restserver/internalapi_linux_test.go	Updates tests to verify node IP usage and legacy rule deletion
cns/restserver/internalapi_linux.go	Implements legacy iptables deletion and changes SNAT target to node IP
cns/fakes/iptablesfake.go	Adds mock implementation for legacy iptables testing

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[QxBytes]

/azp run Azure Container Networking PR

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