Refactor DataManagerImpl and update README

- Refactored `DataManagerImpl` to use `FileInputStream` instead of `Files.newInputStream` for reading files.
- Updated the `README` to replace `registerOnDataChangeListener` with `addDataObserver` in the methods overview.

Add KotlinxSerializationConverter class for enhanced serialization su…

…pport

Some improvements.

Merge remote-tracking branch 'origin/master'

Update version name to 3 and version code to 1.3

Changed Files.newInputStream(file.toPath()) to new FileInputStream(file)

Add flexible saveData method for generic object types

close the bufferedReader

Create README.md

Fixed bug