Create Plugin Manager / Plugins for RF

Problem/Motivation

There's no bridge between Drupal and React Flow.

Proposed resolution

Define a plugin manager and a plugin base to handle the basics: inputs, schema/configs, outputs - basically a plugin would create a special custom node, that will automagically know how to map itself into RF JS environment.

This might also require custom endpoints (or clever ways to feed stuff into a RF UI).

Do note that eventually we won't use /rf/ endpoint -> but Modeler API will automatically tell us where we should expose the UI (it will be embedded in the Drupal's admin theme) - this might be a future issue of its own, we'll see.