Improve 'ipSliceDifference' function memory footprint

[Didainius]

Methods NsxtEdgeGateway.GetAllUnusedExternalIPAddresses and NsxtEdgeGateway.GetUnusedExternalIPAddresses perform computations based on netip.Addr package.

The internal function ipSliceDifference is used to allocate a new slice to hold the final results. However, the operations become quite expensive memory wise having big subnets in Edge Gateway. Optimizing this function reduced memory consumption twice (see example below)

Example information

One example was allocation /8 IPv4 subnet (16777213 IPs)

Before optimization 2 operations consumed such an amount of memory (getAllUnusedExternalIPAddresses - 7703.94MB)

After optimization 2 operations of getAllUnusedExternalIPAddresses consume such amount (after 3853.80MB vs before 7703.94MB):

[adambarreiro]

LGTM! Thanks for answering the doubts, it helped a lot.