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feat: expose subnet refresh interval as configurable setting

Adds SUBNET_REFRESH_INTERVAL (--subnet-refresh-interval, default 1m)
to control how often Karpenter refreshes subnet data from EC2. This
allows operators running large fleets in shared accounts to reduce
DescribeSubnets API call volume by accepting longer subnet staleness.

All three subnet provider caches (subnet list, available IP counts,
and public IP association) use the same interval since they are all
populated from a single DescribeSubnets response.