lisa.target: Add Target.from_conf_setup()

Uses the TargetConf "setup" key, which is a list of functions taking a
target as parameter and returing a context manager.

Make use of that key in the new from_conf_setup() factory that will
stream Target objects while applying the setup context manager from the
"setup" key.

De-annotate Target.from_conf(), since from_conf_setup() is more suited
to exekall.