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/secrets/helpers.go

@@ -0,0 +1,31 @@
+package secrets
+
+import (
+	"os"
+
+	"github.com/qdm12/gluetun/internal/configuration/sources/files"
+)
+
+func readSecretFileAsStringPtr(secretPathEnvKey, defaultSecretPath string) (
+	stringPtr *string, err error) {
+	path := os.Getenv(secretPathEnvKey)
+	if path == "" {
+		path = defaultSecretPath
+	}
+	return files.ReadFromFile(path)
+}
+
+func readSecretFileAsString(secretPathEnvKey, defaultSecretPath string) (
+	s string, err error) {
+	path := os.Getenv(secretPathEnvKey)
+	if path == "" {
+		path = defaultSecretPath
+	}
+	stringPtr, err := files.ReadFromFile(path)
+	if err != nil {
+		return "", err
+	} else if stringPtr == nil {
+		return "", nil
+	}
+	return *stringPtr, nil
+}