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/settings/helpers/obfuscate.go

@@ -0,0 +1,25 @@
+package helpers
+
+func ObfuscateWireguardKey(fullKey string) (obfuscatedKey string) {
+	const minKeyLength = 10
+	if len(fullKey) < minKeyLength {
+		return "(too short)"
+	}
+
+	lastIndex := len(fullKey) - 1
+	return fullKey[0:2] + "..." + fullKey[lastIndex-2:]
+}
+
+func ObfuscatePassword(password string) (obfuscatedPassword string) {
+	if password != "" {
+		return "[set]"
+	}
+	return "[not set]"
+}
+
+func ObfuscateData(data string) (obfuscated string) {
+	if data != "" {
+		return "[set]"
+	}
+	return "[not set]"
+}