feat: add fields in pod network crd for prefix block mode for swiftv2

[kmurudi]

Reason for Change:
Adding new fields that would be required in podnetwork CRD for enabling prefix block IP allocation to pods in swiftv2 flow. Design doc - https://microsoft.sharepoint.com/:w:/t/ContainerNetworking/EbbITCQaW91FrROW1XQ74ewBT8Sp1GfdJ9z5DcuksFH1_g?e=RaqQPT

Requirements:

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

Notes:

[Copilot]

Pull Request Overview

This PR adds new fields to the PodNetwork CRD to support prefix block IP allocation mode for SwiftV2 flow. The changes introduce a subnet allocation mode configuration with two options: Single (default) and PrefixBlock, along with a configurable prefix block size.

• Added SubnetAllocationMode enum type with Single and PrefixBlock options
• Added subnetAllocationMode field to PodNetworkSpec with Single as default
• Added prefixBlockAllocationSize field to PodNetworkStatus with default value of 16

Reviewed Changes

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

File	Description
crd/multitenancy/manifests/multitenancy.acn.azure.com_podnetworks.yaml	Updated CRD manifest with new subnetAllocationMode and prefixBlockAllocationSize fields
crd/multitenancy/api/v1alpha1/podnetwork.go	Added SubnetAllocationMode type definition and new fields to PodNetworkSpec and PodNetworkStatus structs

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[kmurudi]

/azp run Azure Container Networking PR

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