Optimize packing subnetConversionData for initialValidatorSet

[iansuvak]

Context and scope
#547 is merged but the subnetConversionData packing is created by recursively calling abi.encodePacked on the already packed data for each iteration over the initialValidators array. We should make it allocate the data once and copy packed initial validator data into it instead of allocating a new growing bytes array on each iteration

Discussion and alternatives
Open questions