Root config: Add env config (#6400)

https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4880198100
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4916892425
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4929136670

common/platform/env_reload.go

@@ -0,0 +1,40 @@
+package platform
+
+import (
+	"errors"
+	"sync"
+)
+
+var envReloadRegistry = struct {
+	sync.RWMutex
+	handlers []func() error
+}{}
+
+// RegisterEnvReload registers an environment reload handler and runs it once
+// immediately so package defaults keep the same behavior as init-time reads.
+func RegisterEnvReload(handler func() error) {
+	if handler == nil {
+		return
+	}
+	envReloadRegistry.Lock()
+	envReloadRegistry.handlers = append(envReloadRegistry.handlers, handler)
+	envReloadRegistry.Unlock()
+	if err := handler(); err != nil {
+		panic(err)
+	}
+}
+
+// ReloadEnvSettings refreshes all registered environment-backed package state.
+func ReloadEnvSettings() error {
+	envReloadRegistry.RLock()
+	handlers := append([]func() error{}, envReloadRegistry.handlers...)
+	envReloadRegistry.RUnlock()
+
+	var errs []error
+	for _, handler := range handlers {
+		if err := handler(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}