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.

Start at the WebView configuration entry point and trace how the default/device user-agent is read or overridden. The change is complete when configuration can append a string such as ` my_app/1.0` while preserving the existing device, OS, browser, and version information.

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