chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/constants/providers.go

@@ -0,0 +1,27 @@
+package constants
+
+func AllProviders() []string {
+	return []string{
+		Custom,
+		Cyberghost,
+		Expressvpn,
+		Fastestvpn,
+		HideMyAss,
+		Ipvanish,
+		Ivpn,
+		Mullvad,
+		Nordvpn,
+		Perfectprivacy,
+		Privado,
+		PrivateInternetAccess,
+		Privatevpn,
+		Protonvpn,
+		Purevpn,
+		Surfshark,
+		Torguard,
+		VPNUnlimited,
+		Vyprvpn,
+		Wevpn,
+		Windscribe,
+	}
+}