Add enum types for messageType and validate user data when / before constructing actions

[johannesloher]

Currently, the messageTypeof MQTT actions is simply a string. We should add an enum type containing all the possible values and validate the user input (action definition) against that when or before constructing an action. (see https://github.com/MaibornWolff/alt-core-js/pull/62#discussion_r334917536)