[medusa-interfaces] : Adds decorator functionality to BaseService (#39)

Plugins and projects can add decorators in services. E.g. if a plugin needs to load some additional information on carts the plugin can register a decorator via: `cartService.addDecorator(someFunc)` which will be available later through `cartService.runDecorators()`.

packages/medusa-plugin-permissions/src/api/middlewares.js

@@ -1,14 +0,0 @@
-// This middleware is injected to ensure authorization of requests
-// Since this middleware uses the user object on the request, this should be
-// injected after authentication in the core middleware, hence we name
-// the middleware postAuth.
-export default postAuth = () => {
-  return (err, req, res, next) => {
-    const permissionService = req.scope.resolve("permissionService")
-    if (permissionService.hasPermission(req.user, req.method, req.path)) {
-      next()
-    } else {
-      res.status(422)
-    }
-  }
-}