Instead of special-casing syncronous commands, add a decorator similar
to @async_command(), which can be used to decorate extension methods
for execution as a synchronous command.
Change-Id: I1b27f179f667cb065bcffd71ae0f303b05d33b82
Move extensions under an ironic_python_agent.extensions module. This
change also moves the @async_command() decorator into the base
extension module.
Change-Id: I4021fcc33a30f3460a31bca44a4bf776cd53d488