Add onStart property in AutoButton

[alanko0511]

This PR introduces a new onStart callback on AutoButton. This callback is called when the button is clicked and before the action is run. Useful for changing form state or other UI interactions when the button is clicked.

PR Checklist

• Important or complicated code is tested
• Any user facing changes are documented in the Gadget-side changelog
• Any immediate changes are slated for release in Gadget via a generated package dependency bump
• Versions within this monorepo are matching and there's a valid upgrade path

[infiton]

should it be onClick?