v2.2.0

• Please refer to https://fzyzcjy.github.io/flutter_rust_bridge/guides/miscellaneous/whats-new for what's changed in V2.
• Fix geneartion of async functions when default_dart_async: false ; Update frb_example/integrate_third_party #2190 (thanks @Larpoux)
• Don't try to cleanup folders that don't exist #2210 (thanks @Markus43)
• Allow users to override the automatic function rename #2227
• Export more objects to allow custom thread pool without using flutter_rust_bridge::for_generated namespace #2228