[webview_flutter] Address privacy manifest requirement

This plugin is listed on https://developer.apple.com/support/third-party-SDK-requirements/ as requiring a privacy manifest. It's not currently clear why; we'll need to clarify why it's there and then either
- add a privacy manifest, and figure out how to embed it, or
- have it removed from the list if it's there in error.

See https://github.com/flutter/flutter/issues/131940 for context.