sqflite ffi async

sqflite based ffi implementation. Based on sqlite_async. Thanks to PowerSync

• Works on Linux, MacOS and Windows on both Flutter and Dart VM.
• Works on iOS and Android (using sqlite3_flutter_libs - Thanks to Simon Binder)

Caveats

This is a work in progress.

• Readonly support is provided by sqflite_common_ffi
• In memory support is provided by sqflite_common_ffi
• Logger is not supported yet.
• Single/multiple instance is ignored as sqflite_async handles opening/closing.