✨ Add support for AWS Regional NAT Gateway

[moko-poi]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR implements support for AWS Regional NAT Gateways, a new feature announced by AWS in November 2025. Regional NAT Gateways provide automatic multi-AZ expansion with simplified network architecture and enhanced security compared to traditional zonal NAT Gateways.

Key Changes:

• Added natGatewayAvailabilityMode field to VPCSpec with enum validation (Zonal/Regional)
• Updated AWS SDK ec2 service from v1.233.0 to v1.275.1 to support new Regional NAT Gateway parameters
• Implemented reconcileRegionalNatGateway() for creating and managing a single NAT Gateway across all AZs
• Implemented createRegionalNatGateway() using VpcId and AvailabilityMode parameters
• Updated getNatGatewayForSubnet() to support routing through regional NAT Gateways
• Generated updated CRD manifests with new field and kubebuilder validation
• Added comprehensive unit tests covering creation, reconciliation, and subnet routing

Benefits:

• Simplified setup: Single NAT Gateway ID across all Availability Zones
• Enhanced security: No public subnets required for hosting NAT Gateways
• Automatic high availability: NAT Gateway automatically expands/contracts based on workload presence
• Higher limits: Up to 32 IP addresses per AZ (vs 8 for zonal)
• Cost optimization: Eliminates cross-AZ data transfer costs in some scenarios
• Backward compatible: Defaults to Zonal mode, existing clusters unaffected

Example Usage:

apiVersion: infrastructure.cluster.x-k8s.io/v1beta2 kind: AWSCluster metadata: name: my-cluster spec: region: us-east-1 network: vpc: natGatewayAvailabilityMode: Regional # New field, defaults to Zonal

Which issue(s) this PR fixes:
Fixes #5790

Special notes for your reviewer:

• Regional NAT Gateways are available in all commercial AWS Regions except AWS GovCloud (US) and China Regions
• The implementation maintains full backward compatibility - existing clusters continue to use zonal NAT Gateways
• Migration from zonal to regional mode would require recreating the NAT Gateway (not covered in this PR)
• The AWS SDK update (v1.233.0 → v1.275.1) was necessary to access the AvailabilityMode and VpcId parameters for CreateNatGatewayInput
• Test coverage includes both successful creation scenarios and error handling

Checklist:

• squashed commits
• includes documentation
• includes emoji in title ✨
• adds unit tests
• adds or updates e2e tests

Release note:

Add support for AWS Regional NAT Gateways. Users can now configure `natGatewayAvailabilityMode: Regional` in VPCSpec to create a single NAT Gateway that automatically expands across all Availability Zones, providing simplified setup and enhanced security. Defaults to Zonal mode for backward compatibility. 

[k8s-ci-robot]

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