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/files/helpers.go

@@ -0,0 +1,31 @@
+package files
+
+import (
+	"io"
+	"os"
+)
+
+// ReadFromFile reads the content of the file as a string.
+// It returns a nil string pointer if the file does not exist.
+func ReadFromFile(filepath string) (s *string, err error) {
+	file, err := os.Open(filepath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil //nolint:nilnil
+		}
+		return nil, err
+	}
+
+	b, err := io.ReadAll(file)
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+
+	if err := file.Close(); err != nil {
+		return nil, err
+	}
+
+	content := string(b)
+	return &content, nil
+}