Enhance Plugin API

[aschauerhuber]

• [ ] add high level methods for schema management
• [ ] add mechanism to apply changes to plugin data (nodes)
• [ ] add dedicated plugin schema for plugin root node (instead of folder schema)
• [ ] add mesh core enhancement which creates plugins root node for projects (during plugin deployment), with provided core 'plugins' schema (in addition to folder schema)
  • [ ] provide field to identify last used plugin version, plugin uuid (e.g. for downgrade checks, and purging)
• [ ] add method to retrieve plugin root node
• [ ] bootstrap plugin for onboarding developers
• [ ] enhance plugin deployment mechanism with plugin configuration (json), extra endpoint to update plugin configuration
• [ ] enhance cli to support plugin configuration changes
• [ ] step by step guide
• [ ] docs on how to develop plugins for multiple projects

[Jotschi]

• unified error handling
• [x] think of a way to extend / override / change plugin code via another plugin (This is now possible via extensions)