Add `--target-imgref` option to bootc switch

Following a customer case where they had lost connectivity to their registry and manually scp'd and oci image followed by podman load then `bootc switch --transport containers-storage ` which ended up not working out for upgrades as bootc would start fetching upgrades from containers-storage.

Having a `--target-imgref` option for switch would be extremely helpful here

Start at the `bootc switch` command entry point and inspect how the existing `--transport` option determines the image source. Trace the switch and upgrade-source handling, then verify that `--target-imgref` allows a locally loaded image to be used as the target while future upgrades continue fetching from the intended registry reference.

Written by the indexing model from the issue text.