Add dropped packet counters to network statistics

[JaewonHur]

Expose the per-interface rx/tx dropped packet counts from
rtnl_link_stats64 through container statistics. Packets can be dropped
by the device (e.g. a full rx ring or tx queue) without being counted
as errors, so receivedErrors/transmittedErrors alone don't show this
loss.

• Add receivedDropped and transmittedDropped to the NetworkStats proto
  message (fields 8 and 9) and regenerate the Swift bindings.
• Populate them in vminitd's statistics RPC from the netlink link stats.
• Surface them on ContainerStatistics.NetworkStatistics. The new
  initializer parameters default to 0, so existing callers keep
  compiling.
• Add an integration test covering the new counters.