fix(ec2): IPAM allocated subnets cannot split more than 256 times

[rix0rrr]

Because of IPAM allocation, we can't know the parent CIDR at synth time, so we cannot calculate the CIDR split at synth time either.

This forces us to rely on the { Fn::Cidr } function provided by CloudFormation. For resource consumption reasons, this function is limited to splitting any range into at most 256 subranges, which means the IPAM allocated VPC cannot split into more subranges either.

This PR adds a recursive split feature: if we need to split an CIDR range more than 256 times, we will do multiple splits:

Fn.select(300, Fn.cidr(range, 4096, 4)) // <-- illegal

// ==

Fn.select(44, Fn.cidr(Fn.select(1, Fn.cidr(range, 4, 12)), 256, 4))

Fixes #25537.

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