Allow using IPv6 blocks smaller than /56 for VPCs (e.g. /60 from IPAM)

[rootops]

Hi 👋

In the current module, IPv6 subnets are calculated using this logic:

cidrsubnet(aws_vpc.this[0].ipv6_cidr_block, 8, var.private_subnet_ipv6_prefixes[count.index])

This assumes the VPC always receives a /56 IPv6 block, and splits it into /64 subnets (which AWS requires).

However, when using custom IPAM pools, it's valid and sometimes desirable to assign smaller blocks like /60 to each VPC (e.g. to fit multiple VPCs into a /48 pool).

Because newbits is hardcoded to 8, the module doesn't support allocating a /60 to a VPC and deriving subnets correctly from it.
Suggestion :
Make the newbits value configurable (defaulting to 8) to support use cases where each VPC only gets a /60 block.

Regards,
MB