[pluginApi] expose support flags

With elastic#5871 we removed support for configuration-based autoloading and moved to require-based autoloading. This change is difficult for plugin authors to make in a backwards compatible way so I thought
it would make sense to start helping plugin authors know which features are/are not available to them with a simple collection of "supports" flags. For now it just shows that `autoload: false`, so that
plugins can expose an alternate main file which includes the autoloads they require

src/server/plugins/PluginApi.js

@@ -8,6 +8,10 @@ module.exports = class PluginApi {
     this.rootDir = kibana.rootDir;
     this.package = require(join(pluginPath, 'package.json'));
     this.Plugin = Plugin.scoped(kibana, pluginPath, this.package);
+
+    this.support = {
+      autoload: false
+    };
   }
 
   get uiExports() {