contrib: Add script to start backport
contrib: Add script to start backport

This script will pull the latest changes from the 'origin' repository,
create a new branch, and run 'check-stable' to list all changes that
need to be backported, leaving behind a summary file for the changes.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium,cilium-team/cilium