v0.9.0

• Update to objectbox-c 0.11.0
• Update to objectbox-android 2.8.0
• Change box.get(id) to return null instead of throwing when trying to read a non-existent object.
• Change the generator to skip read-only fields (getters)
• Add SyncClient to enable the new ObjectBox Sync
• Add "empty" query support using box.query().
  (thanks @Buggaboo for #132)
• Expose lib/observable.dart to avoid linter isues in apps using objectbox.
  (thanks @cmengler for bringing this up in #141)
• Documentation and examples updates & improvements.
• Switch C-API binding code generation to package ffigen.