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/files.go

@@ -0,0 +1,31 @@
+package helpers
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+var (
+	ErrFileDoesNotExist = errors.New("file does not exist")
+	ErrFileRead         = errors.New("cannot read file")
+	ErrFileClose        = errors.New("cannot close file")
+)
+
+func FileExists(path string) (err error) {
+	path = filepath.Clean(path)
+
+	f, err := os.Open(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("%w: %s", ErrFileDoesNotExist, path)
+	} else if err != nil {
+		return fmt.Errorf("%w: %s", ErrFileRead, err)
+	}
+
+	if err := f.Close(); err != nil {
+		return fmt.Errorf("%w: %s", ErrFileClose, err)
+	}
+
+	return nil
+}