CLB 2nd PR: Add DiffTracker core types, state management, and sync operations

[georgeedward2000]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR introduces the DiffTracker core package — the foundational types, state management, and diff computation logic for the Container Load Balancer (CLB) feature.

The DiffTracker maintains two parallel state representations:

• K8s State: Services, Egresses, and Nodes (with Pods and their inbound/outbound identities)
• NRP State: LoadBalancers, NATGateways, and Locations (with Addresses and service references)

It provides operations to mutate both states and compute the diff between them, determining what needs to be synced to Azure.

Package contents (pkg/provider/difftracker/):

File	Content
types.go	Core enums (Operation, UpdateAction, SyncStatus), K8s/NRP state model types, DiffTracker struct, input types, sync result types
config.go	Config struct with Azure subscription, resource group, location, and ServiceGateway fields
difftracker.go	InitializeDiffTracker() constructor with validation
k8s_state_updates.go	K8s state mutations: services, egresses, endpoints, pods
nrp_state_updates.go	NRP state mutations: LoadBalancers, NATGateways, location/address updates
sync_operations.go	Diff computation between K8s and NRP states (GetSyncOperations, GetSyncLoadBalancerServices, GetSyncNRPNATGateways, GetSyncLocationsAddresses)
util.go	Enum serialization, DeepEqual, Equals comparison methods

Dependency changes (pkg/util/sets/string.go):

• Added Equals() method to IgnoreCaseSet for set comparison
• Added MarshalJSON() method to IgnoreCaseSet for JSON serialization

Test coverage:

• difftracker_test.go — 14 test functions covering initialization, state updates, sync operations, and end-to-end scenarios
• util_test.go — 11 test functions covering enum serialization, JSON round-trips, HasPods/HasIdentities helpers, DeepEqual, Equals, and Config.Validate()
• string_test.go — Added TestEquals (8 sub-tests) for the new IgnoreCaseSet.Equals() method

This is PR 2 in the CLB series, building on the configuration and constants from PR 1 (#9775). Engine-layer code (async processing, updaters, finalizers, metrics, Azure operations) and ServiceGateway API DTOs/mappers are intentionally excluded and will follow in subsequent PRs.

Which issue(s) this PR fixes:

Part of CLB (Container Load Balancer) feature implementation.

Special notes for your reviewer:

• The isServiceReady() function currently checks NRP state only. In the engine PR (PR 3), it will be extended to also check pendingServiceOps for StateCreated status.
• A DEFERRED_CODE_TRACKER.md is maintained in the source repo tracking what was excluded and which future PR will add it back.
• Depends on PR First CLB batch of changes #9775 (First CLB batch — config, constants).

Does this PR introduce a user-facing change?

NONE

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: georgeedward2000
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