feat(pytauri)!: add BuilderArgs::setup to support setup hook

[WSH032]

Summary

Added:

• add BuilderArgs::setup
• BuilderArgs::context now can be Position and Keyword arguments
• add PyAppHandleExt::get_or_init_py_app_handle, and the methods return &Py<AppHandle> instead of impl Deref<Target = Py<AppHandle>> now.

Docs:

• example/nicegui-app:
  • Use BuilderArgs.setup for initialization instead of listening to the RunEvent.Ready event.
  • Rewrite the FrontServer startup/shutdown event hook logic.
  • Modularize the code.

BREAKING CHANGE:

• trait PyAppHandleExt is now sealed and no longer has generic parameters.

Checklist

• I've read CONTRIBUTING.md.
• I understand that this PR may be closed in case there was no previous discussion/issue. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation and/or CHANGELOG.md accordingly.