Add the option to modify the browser's default "user-agent" in settings.

[Zentrox]

Users who are a little more “technical” like to customize the way the browser identifies itself to the web, and one of these ways is through the “user-agent”. In Brave for desktop this can be easily done using an extension from the store, but in the Android version there is no support for extensions (for now), so how about we add this user-agent option?