feat: exposing `onMessage` as a prop

Thanks for your work on this library! Is it possible to expose [`onMessage`](https://github.com/chatwoot/chatwoot-react-native-widget/blob/cd280c2/src/WebView.js#L89) as a callback prop similar to how on the web, you can listen to Chatwoot messages and hook in your own custom functionality?

Start in src/WebView.js at the onMessage handling referenced in the issue, then inspect how component props are defined and passed through the widget. Expose the callback as a React Native prop while preserving the existing message handling, and verify that consumers can receive Chatwoot messages for custom functionality.

Written by the indexing model from the issue text.