Add beforeOpen callback to native factories

[simolus3]

Sometimes, it's convenient to run an async setup step. For example, the /tmp directory is inaccessible on older Android versions and one might want to use path_provider to configure sqlite3_temp_directory to point to an app-specific temporary directory.

Of course, one could simply do that before using databases:

void main() {
  sqlite3.tempDirectory = await findTemporaryDirectory();
  final db = PowerSyncDatabase(...);
}

But then you need an async suspension before your app can even reference a database, which is annoying. Since databases are opened asynchronously internally, this adds an async callback for that instead.

cc @davidmartos96