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/env/dot.go

@@ -0,0 +1,31 @@
+package env
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+)
+
+func (r *Reader) readDoT() (dot settings.DoT, err error) {
+	dot.Enabled, err = envToBoolPtr("DOT")
+	if err != nil {
+		return dot, fmt.Errorf("environment variable DOT: %w", err)
+	}
+
+	dot.UpdatePeriod, err = envToDurationPtr("DNS_UPDATE_PERIOD")
+	if err != nil {
+		return dot, fmt.Errorf("environment variable DNS_UPDATE_PERIOD: %w", err)
+	}
+
+	dot.Unbound, err = readUnbound()
+	if err != nil {
+		return dot, err
+	}
+
+	dot.Blacklist, err = r.readDNSBlacklist()
+	if err != nil {
+		return dot, err
+	}
+
+	return dot, nil
+}