workflow: Add workflow verifing integration with the upstream Zephyr

During the workflow a Zephyr repository is checked out (by default
main, but any version can be given when workflow is triggered
manually) and west update is run according to Zephyr's manifest.
However, the MCUboot version is then replaced by checking out
MCUboot main (triggered by push or cron), PR's head (triggered by
a PR to MCUboot) or custom SHA (manual trigger). Next, twister is
called for tests/builds in locations given in the workflow
(test_paths).

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>