[DAEMON-309] Add appcd plugin API version

The Appc Daemon's plugin system allows plugins to specify which appcd version they are compatible with, however we have to ship new plugins for each major plugin version whenever we bump the appcd major version regardless if the plugin system actually changed. Since the plugin system rarely changes, we shouldn't couple plugins with the appcd version, but rather a separate plugin API version. For example, the iOS appcd plugin 1.x is compatible with every appcd version, but iOS appcd plugin 2.x requires appcd version 3.x because of changes to appcd-detect that is passed into each plugin.