Added flags field to Network Subnets

[daschott]

Subnets have a flags field that is needed to fix issue for network rehydration when GatewayAddress == ManagementIP.

Customers need to be able to use the Flag "DoNotReserveGatewayAddress" in order to support rehydration for the above scenario.

[daschott]

Have tested this change and verified it can be used to resolve the issue

[Copilot]

Pull Request Overview

This PR adds support for subnet flags to enable proper network rehydration when the gateway address equals the management IP. This addresses a specific customer scenario where the "DoNotReserveGatewayAddress" flag is needed for successful network rehydration.

Key changes:

• Introduces SubnetFlags type and constants for flag management
• Adds a Flags field to the Subnet struct to support subnet-level configuration options

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[princepereira]

KubeProxy is currently the only consumer of hnslib. The long-term plan is to deprecate the hcn code in hcsshim and migrate it to hnslib, but that transition will take some time. Until then, any hcn changes added to hnslib should also be reflected in hcsshim and vice-versa.

[daschott]

KubeProxy is currently the only consumer of hnslib. The long-term plan is to deprecate the hcn code in hcsshim and migrate it to hnslib, but that transition will take some time. Until then, any hcn changes added to hnslib should also be reflected in hcsshim and vice-versa.

Equivalent hcsshim PR: microsoft/hcsshim#2525