refactor: refactor SDK using flutter_web_auth_2

[simeng-li]

Summary

Refactor @logto/dart SDK using the new flutter_web_auth_2 library.

Context

This PR refactors the Logto Dart SDK using the new flutter_web_auth_2 package to handle the OAuth2 user authorization flow.

The SDK originally used the flutter_web_auth package to handle the OAuth2 user authorization flow. However, this outdated package is no longer maintained. We have received multiple reported issues with the package, which does not work properly with some of the latest platforms and devices. Hence, we refactor the SDK to use the new flutter_web_auth_2 package.

Updates

These updates will not affect the SDK API and user experiences. Major fixes and improvements are:

1. Fix the namespace missing issue when building with the latest Gradle version on Android. (bug: Can`t build android apk. #75 )
2. Fix the issue that the webview is not closing after the user completes the OAuth2 authorization flow on Android. (doc: social logins signups redirect loop on android when android:taskAffinity="" #60)
3. The SDK now supports web applications with the latest flutter_web_auth_2 package. (feature request: consider using flutter_web_auth_2 #61 )
4. Fix the issue on Android that the sign-in session is not cleared after the user signs out. The preferEphemeral parameter does not work on Android. Even with the ephemeralIntentFlags specified, the webview still preserves the Logto sign-in session on Android. As a workaround, for Android and Web platforms, redirect the user to the end_session_endpoint to clear the sign-in session after the user signs out manually.

Breaking change

logtoClient.signOut method now requires a redirectUri parameter. For iOS platform, this parameter is useless, but for Android and Web platforms which require an additional end_session request to clean up the sign-in session, this parameter will be used as the post_logout_redirect_uri parameter in the end_session request.

Testing

Test locally using iOS, Android, and Chrome simulators.

Checklist

• .changeset
• unit tests
• integration tests
• necessary TSDoc comments