[EASY] Add UI option to pass through custom `EXO_LIBP2P_NAMESPACE` from Swift

[JakeHillion]

EXO supports multiple p2p namespaces to split up clusters. This is usable when building from source by providing the EXO_LIBP2P_NAMESPACE environment variable when running from source, but when using the macOS app this is hardcoded to the application version.

Add an option in the macOS UI to extend this variable with a custom parameter. This will allow splitting machines that can see each other into multiple clusters.