[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.

combined/cmd/config.go

@@ -5,20 +5,18 @@ import (
 	"fmt"
 	"net"
 	"net/netip"
-	"os"
 	filePath "path/filepath"
 	"strings"
 	"time"
 
 	log "github.com/sirupsen/logrus"
-	"gopkg.in/yaml.v3"
 
+	nbconfig "github.com/netbirdio/netbird/management/internals/server/config"
 	"github.com/netbirdio/netbird/management/server/idp"
 	"github.com/netbirdio/netbird/management/server/types"
 	"github.com/netbirdio/netbird/util"
+	configloader "github.com/netbirdio/netbird/util/config"
 	"github.com/netbirdio/netbird/util/crypt"
-
-	nbconfig "github.com/netbirdio/netbird/management/internals/server/config"
 )
 
 // CombinedConfig is the root configuration for the combined server.
@@ -440,26 +438,17 @@ func (c *CombinedConfig) autoConfigureClientSettings(exposedProto, exposedHost,
 	}
 }
 
-// LoadConfig loads configuration from a YAML file
+// LoadConfig loads the combined server configuration.
 func LoadConfig(configPath string) (*CombinedConfig, error) {
-	cfg := DefaultConfig()
-
-	if configPath == "" {
-		return cfg, nil
-	}
-
-	data, err := os.ReadFile(configPath)
+	cfg, err := configloader.Load(configPath, DefaultConfig(), configloader.Options{
+		TagName:      "yaml",
+		AllowMissing: configPath == "",
+	})
 	if err != nil {
-		return nil, fmt.Errorf("failed to read config file: %w", err)
+		return nil, err
 	}
 
-	if err := yaml.Unmarshal(data, cfg); err != nil {
-		return nil, fmt.Errorf("failed to parse config file: %w", err)
-	}
-
-	// Populate internal configs from server settings
 	cfg.ApplySimplifiedDefaults()
-
 	return cfg, nil
 }