cilium: ipsec, refactor reading IPSec keys to support io.Reader
cilium: ipsec, refactor reading IPSec keys to support io.Reader

Refactor code to read keys from io.Reader with a wrapper to use a file
as the io.Reader. This simplifies the unit tests but also allows us to
read keys from other readers if needed.

Signed-off-by: John Fastabend <ee461b60978bea1c76b3018c0b89685175b248e2@gmail.com>

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