* Add XPackExtension: an api class (like Plugin in core) for what a x-pack extension can do.
* Add XPackExtensionCli: a cli tool for adding, removing and listing extensions for x-pack.
* Add XPackExtensionService: loading of jars from pluginsdir/x-pack/extensions, into child classloader.
* Add bin/x-pack/extension script, similar to plugin cli, which installs an extension into pluginsdir/x-pack/extensions.
* Add XPack extension integration test.
Fixedelastic/elasticsearch#1515
Original commit: elastic/x-pack-elasticsearch@130ba03270