Support multiple subnets in custom networking mode

[adammw]

What would you like to be added:
Change the schema of Custom Networking mode configuration CRDs (ENIConfig) to support specifying multiple subnets. At runtime, use the same logic as Custom Networking to have each interface attachment pick the subnet with the most number of free IPs.

Why is this needed:
Subnet discovery is incompatible with Custom Networking mode - for full control over which pods on which nodes get attached to which subnets, Custom Networking mode is the only way. However this means we cannot specify multiple subnets and get the CNI plugin to pick the subnet with the most number of free IPs. Adding multiple subnets to the CRD would allow the CNI to choose from a specific list of subnets rather than need to discover the subnets using tags.

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Issue closed due to inactivity.