Standalone Sync API - main project

Being cross-platform and able to sync notes are crucial features of a note-taking app which Joplin implemented very well. Currently, Joplin is on these platforms: desktop, mobile, and terminal, it would be best to separate the current Joplin synchronizer as a standalone syncing library. Such re-implementation can decouple the ecosystem, and allowing more user control over the syncing process. Porting current Joplin's synchronization modules to a separate library. Integrate with some Cloud Providers: Onedrive, Dropbox,... Exposing services to allow more control over syncing process. Adding unit & integration tests for new library.