MM-9674 Add plugin API for publishing custom WebSocket events (#8999)

* Add plugin API for publishing custom WebSocket events

* Add clearer payload comment

* Update comment

plugin/plugintest/api.go

@@ -1,4 +1,4 @@
-// Code generated by mockery v1.0.0. DO NOT EDIT.
+// Code generated by mockery v1.0.0
 
 // Regenerate this file using `make plugin-mocks`.
 
@@ -563,6 +563,11 @@ func (_m *API) LoadPluginConfiguration(dest interface{}) error {
 	return r0
 }
 
+// PublishWebSocketEvent provides a mock function with given fields: event, payload, broadcast
+func (_m *API) PublishWebSocketEvent(event string, payload map[string]interface{}, broadcast *model.WebsocketBroadcast) {
+	_m.Called(event, payload, broadcast)
+}
+
 // RegisterCommand provides a mock function with given fields: command
 func (_m *API) RegisterCommand(command *model.Command) error {
 	ret := _m.Called(command)