[management,signal,proxy,relay,misc] Unify service configuration loading

Service entry points currently resolve defaults, files, environment variables, and flags differently, which makes precedence inconsistent and prevents some services from using config files.

Introduce one Viper-backed loader and migrate Combined, Management, Relay, Signal, and Proxy while preserving compatibility aliases and Management template expansion.

management/cmd/config.go

@@ -0,0 +1,13 @@
+package cmd
+
+import (
+	nbconfig "github.com/netbirdio/netbird/management/internals/server/config"
+	configloader "github.com/netbirdio/netbird/util/config"
+)
+
+func loadManagementConfig(configPath string) (*nbconfig.Config, error) {
+	return configloader.Load(configPath, &nbconfig.Config{Datadir: defaultMgmtDataDir}, configloader.Options{
+		TagName:   "json",
+		Transform: configloader.ExpandEnvTemplate,
+	})
+}