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/configuration/sources/env/provider.go

@@ -0,0 +1,40 @@
+package env
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants"
+)
+
+func (r *Reader) readProvider(vpnType string) (provider settings.Provider, err error) {
+	provider.Name = readVPNServiceProvider(vpnType)
+
+	provider.ServerSelection, err = r.readServerSelection(*provider.Name, vpnType)
+	if err != nil {
+		return provider, fmt.Errorf("cannot read server selection settings: %w", err)
+	}
+
+	provider.PortForwarding, err = readPortForward()
+	if err != nil {
+		return provider, fmt.Errorf("cannot read port forwarding settings: %w", err)
+	}
+
+	return provider, nil
+}
+
+func readVPNServiceProvider(vpnType string) (vpnProviderPtr *string) {
+	s := strings.ToLower(os.Getenv("VPNSP"))
+	switch {
+	case vpnType == constants.OpenVPN &&
+		os.Getenv("OPENVPN_CUSTOM_CONFIG") != "": // retro compatibility
+		return stringPtr(constants.Custom)
+	case s == "":
+		return nil
+	case s == "pia": // retro compatibility
+		return stringPtr(constants.PrivateInternetAccess)
+	}
+	return stringPtr(s)
+}