PLT-7407: Back-end plugins (#7409)

* tie back-end plugins together

* fix comment typo

* add tests and a bit of polish

* tests and polish

* add test, don't let backend executable paths escape the plugin directory

api4/api.go

@@ -150,7 +150,7 @@ func InitApi(full bool) {
 	BaseRoutes.PublicFile = BaseRoutes.Root.PathPrefix("/files/{file_id:[A-Za-z0-9]+}/public").Subrouter()
 
 	BaseRoutes.Plugins = BaseRoutes.ApiRoot.PathPrefix("/plugins").Subrouter()
-	BaseRoutes.Plugin = BaseRoutes.Plugins.PathPrefix("/{plugin_id:[A-Za-z0-9\\_\\-]+}").Subrouter()
+	BaseRoutes.Plugin = BaseRoutes.Plugins.PathPrefix("/{plugin_id:[A-Za-z0-9\\_\\-\\.]+}").Subrouter()
 
 	BaseRoutes.Commands = BaseRoutes.ApiRoot.PathPrefix("/commands").Subrouter()
 	BaseRoutes.Command = BaseRoutes.Commands.PathPrefix("/{command_id:[A-Za-z0-9]+}").Subrouter()