[network] Route PacketCaptureProvider into toolbox

Route fuchsia.net.debug/PacketCaptureProvider into toolbox in
preparation for the upcoming pcap target-side binary that will use it
to enable starting/stopping rolling packet captures.

Bug: 516812186
Change-Id: I00e47c49721d848773cb44aad25d311d24b5c11d
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1706055
Reviewed-by: Claire Gonyeo <cgonyeo@google.com>
Reviewed-by: Cameron Finucane <eep@google.com>
Reviewed-by: Jeff Martin <martinjeffrey@google.com>
Commit-Queue: Tony Gong <gongt@google.com>