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/portforward_test.go

@@ -0,0 +1,19 @@
+package settings
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_PortForwarding_String(t *testing.T) {
+	t.Parallel()
+
+	settings := PortForwarding{
+		Enabled: boolPtr(false),
+	}
+
+	s := settings.String()
+
+	assert.Empty(t, s)
+}