Implement new plugin system using PF4J

[DRSchlaubi]

See #1110

!!! BREAKING CHANGES

Note

Old plugins will continue to work, however it is advised to update to the new system as legacy support should be removed eventually

Spring components with dependencies have to be annotated with @extension

Due to the nature of Bean installation in the new plugin system you might need to tell Lavalink to load certain components first

Example

@Extension(ordinal = 10) // ordinal needs to be higher than the ordinal of VkMusicSourceManager, which is inferred to be 0
@Service
@RestController
public class LavaSrcPlugin {
	private VkMusicSourceManager vkMusic;
}

[DRSchlaubi]

Lets hope this gets merged soon: pf4j/pf4j#595

[DRSchlaubi]

A new branch for the gradle plugin has been created.