Append to user-agent string

When configuring the WebView, it would be really useful to specify a string to append to the default/device user-agent, rather than overriding it completely.

E.g., a use case would be appending ` my_app/1.0` to the end of the user-agent string to indicate that the user is accessing the website through the Flutter app, but still retaining the device/OS/browser/version info from the original user-agent string.