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/default.go

@@ -0,0 +1,93 @@
+package helpers
+
+import (
+	"net"
+	"time"
+
+	"github.com/qdm12/golibs/logging"
+)
+
+func DefaultInt(existing *int, defaultValue int) (
+	result *int) {
+	if existing != nil {
+		return existing
+	}
+	result = new(int)
+	*result = defaultValue
+	return result
+}
+
+func DefaultUint8(existing *uint8, defaultValue uint8) (
+	result *uint8) {
+	if existing != nil {
+		return existing
+	}
+	result = new(uint8)
+	*result = defaultValue
+	return result
+}
+
+func DefaultUint16(existing *uint16, defaultValue uint16) (
+	result *uint16) {
+	if existing != nil {
+		return existing
+	}
+	result = new(uint16)
+	*result = defaultValue
+	return result
+}
+
+func DefaultBool(existing *bool, defaultValue bool) (
+	result *bool) {
+	if existing != nil {
+		return existing
+	}
+	result = new(bool)
+	*result = defaultValue
+	return result
+}
+
+func DefaultString(existing string, defaultValue string) (
+	result string) {
+	if existing != "" {
+		return existing
+	}
+	return defaultValue
+}
+
+func DefaultStringPtr(existing *string, defaultValue string) (result *string) {
+	if existing != nil {
+		return existing
+	}
+	result = new(string)
+	*result = defaultValue
+	return result
+}
+
+func DefaultDuration(existing *time.Duration,
+	defaultValue time.Duration) (result *time.Duration) {
+	if existing != nil {
+		return existing
+	}
+	result = new(time.Duration)
+	*result = defaultValue
+	return result
+}
+
+func DefaultLogLevel(existing *logging.Level,
+	defaultValue logging.Level) (result *logging.Level) {
+	if existing != nil {
+		return existing
+	}
+	result = new(logging.Level)
+	*result = defaultValue
+	return result
+}
+
+func DefaultIP(existing net.IP, defaultValue net.IP) (
+	result net.IP) {
+	if existing != nil {
+		return existing
+	}
+	return defaultValue
+}