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/shadowsocks.go

@@ -0,0 +1,54 @@
+package env
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) readShadowsocks() (shadowsocks settings.Shadowsocks, err error) {
+	shadowsocks.Enabled, err = envToBoolPtr("SHADOWSOCKS")
+	if err != nil {
+		return shadowsocks, fmt.Errorf("environment variable SHADOWSOCKS: %w", err)
+	}
+
+	shadowsocks.Address = r.readShadowsocksAddress()
+	shadowsocks.LogAddresses, err = envToBoolPtr("SHADOWSOCKS_LOG")
+	if err != nil {
+		return shadowsocks, fmt.Errorf("environment variable SHADOWSOCKS_LOG: %w", err)
+	}
+	shadowsocks.CipherName = r.readShadowsocksCipher()
+	shadowsocks.Password = envToStringPtr("SHADOWSOCKS_PASSWORD")
+
+	return shadowsocks, nil
+}
+
+func (r *Reader) readShadowsocksAddress() (address string) {
+	address = os.Getenv("SHADOWSOCKS_LISTENING_ADDRESS")
+	if address != "" {
+		return address
+	}
+
+	// Retro-compatibility
+	portString := os.Getenv("SHADOWSOCKS_PORT")
+	if portString != "" {
+		r.onRetroActive("SHADOWSOCKS_PORT", "SHADOWSOCKS_LISTENING_ADDRESS")
+		return ":" + portString
+	}
+
+	return ""
+}
+
+func (r *Reader) readShadowsocksCipher() (cipher string) {
+	cipher = os.Getenv("SHADOWSOCKS_CIPHER")
+	if cipher != "" {
+		return cipher
+	}
+	// Retro-compatibility
+	cipher = os.Getenv("SHADOWSOCKS_METHOD")
+	if cipher != "" {
+		r.onRetroActive("SHADOWSOCKS_METHOD", "SHADOWSOCKS_CIPHER")
+	}
+	return cipher
+}