Create Interface for plugins to compile against.

[martinheidegger]

Currently plugins are written in typescript. To compile the plugins independently of mycloud and use the dynamic loading of plugins to add them, we need to prepare an interface (proposed name: mycloud-plugin-api) that allows an independent compilations of the plugins.

For a proper implementation the same interface needs to be use by mycloud and the plugin interfaces as discrepancies in the use will cause hard-to-debug runtime problems. That being said, for the start it is possible (though to some degree inadvisable!) to release a stub that contains only the minimal necessary functions for plugins to be extracted without having the entire API replicated in the API.