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/string.go

@@ -0,0 +1,15 @@
+package helpers
+
+func BoolPtrToYesNo(b *bool) string {
+	if *b {
+		return "yes"
+	}
+	return "no"
+}
+
+func TCPPtrToString(tcp *bool) string {
+	if *tcp {
+		return "TCP"
+	}
+	return "UDP"
+}