feat: allow defining custom subnet ip ranges for static agent pools

[pat-s]

Instead of relying on the default ones incrementing from 10.0.x.

This can be useful (and even needed) if certain subnets in this range are already occupied (e.g. by a vswitch connection).

The current approach uses a hardcoded logic to increment from 0, e.g. 10.0, 10.1, 10.2, etc. subnet_ip_range allows overriding a node pool subnet on an individual basis.

[mysticaltech]

@pat-s This is a breaking change. What I'm exploring now is for v3 to actually have one network per nodepool. That will allow us to have up to 100 nodes per nodepool, and not the current limited 100 nodes per cluster. It resonates a little bit with what you are trying to achieve here.

[pat-s]

Why is it breaking? It is optional and only used when specified and otherwise uses the old default incrementing from 10.x per node?

[mysticaltech]

@pat-s Man, subnet ranges are computed very carefully, it's a whole equilibrium. I will need to think about that one more.

[mysticaltech]

@codex review please, and let me know if backward compatible or not.

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. You're on a roll.

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