Lowcoder backend plugin system

This is an ongoing effort to refactor current plugin system based on pf4j library.

Reasoning

create a cleaner and simpler plugin system with clearly defined purpose(s) (new endpoints, new datasource types, etc..)
lowcoder does not need live plugin loading/reloading/unloading/updates, therefore the main feature of pf4j is rendered useless, in fact it adds a lot of complexity due to classloaders used for managing plugins (especially in spring/boot applications)
simpler and easier plugin detection - just a jar with a class implementing a common interface (be it a simple pojo project or a complex spring/boot implementation)

How it works

The main entrypoint for plugin system is in lowcoder-server module with class org.lowcoder.api.framework.configuration.PluginConfiguration
It creates:

LowcoderPluginManager bean which is responsible for plugin lifecycle management
Adds plugin defined endpoints to lowcoder by creating pluginEndpoints bean
TODO: Adds plugin defined datasources to lowcoder by creating pluginDatasources bean

lowcoder-plugin-api library

This library contains APIs for plugin implementations.
It is used by both, lowcoder API server as well as all plugins.

PluginLoader

The sole purpose of a PluginLoader is to find plugin candidates and load them into VM.
There is currently one implementation that based on paths - PathBasedPluginLoader, it:

looks in folders and subfolders defined in application.yaml - entries can point to a folder or specific jar file. If a relative path is supplied, the location of lowcoder API server application jar is used as parent folder (when run in non-packaged state, eg. in IDE, it uses the folder where ServerApplication.class is generated)

common:
  plugin-dirs:
  - plugins
  - /some/custom/path/myGreatPlugin.jar

finds all jar(s) and inspects them for classes implementing LowcoderPlugin interface
instantiates all LowcoderPlugin implementations

LowcoderPluginManager

The main job of plugin manager is to:

register plugins found and instantiated by PluginLoader
start registered plugins by calling LowcoderPlugin.load() method
create and register RouterFunction(s) for all loaded plugin endpoints
TODO: create and register datasources for all loaded plugin datasources

Plugin project structure

Plugin jar can be structured in any way you like. It can be a plain java project, but also a spring/boot based project or based on any other framework.

It is composed from several parts:

class(es) implementing LowcoderPlugin interface
class(es) implementing PluginEndpoint interface, containing endpoint handler functions marked with @EndpointExtension annotation. These functions must obey following format:

	@EndpointExtension(uri = <endpoint uri>, method = <HTTP method>)
	public EndpointResponse <handler name>(EndpointRequest request) 
	{
	   ... your endpoint logic implementation		
	}

TODO: class(es) impelemting LowcoderDatasource interface

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