[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.

util/file.go

@@ -1,7 +1,6 @@
 package util
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -10,10 +9,10 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
-	"strings"
-	"text/template"
 
 	log "github.com/sirupsen/logrus"
+
+	configloader "github.com/netbirdio/netbird/util/config"
 )
 
 func WriteBytesWithRestrictedPermission(ctx context.Context, file string, bs []byte) error {
@@ -233,8 +232,6 @@ func ListFiles(dir, pattern string) ([]string, error) {
 
 // ReadJsonWithEnvSub reads JSON config file and maps to a provided interface with environment variable substitution
 func ReadJsonWithEnvSub(file string, res interface{}) (interface{}, error) {
-	envVars := getEnvMap()
-
 	f, err := os.Open(file)
 	if err != nil {
 		return nil, err
@@ -246,19 +243,12 @@ func ReadJsonWithEnvSub(file string, res interface{}) (interface{}, error) {
 		return nil, err
 	}
 
-	t, err := template.New("").Parse(string(bs))
+	output, err := configloader.ExpandEnvTemplate(bs)
 	if err != nil {
-		return nil, fmt.Errorf("error parsing template: %v", err)
+		return nil, err
 	}
 
-	var output bytes.Buffer
-	// Execute the template, substituting environment variables
-	err = t.Execute(&output, envVars)
-	if err != nil {
-		return nil, fmt.Errorf("error executing template: %v", err)
-	}
-
-	err = json.Unmarshal(output.Bytes(), &res)
+	err = json.Unmarshal(output, &res)
 	if err != nil {
 		return nil, fmt.Errorf("failed parsing Json file after template was executed, err: %v", err)
 	}
@@ -266,20 +256,6 @@ func ReadJsonWithEnvSub(file string, res interface{}) (interface{}, error) {
 	return res, nil
 }
 
-// getEnvMap Convert the output of os.Environ() to a map
-func getEnvMap() map[string]string {
-	envMap := make(map[string]string)
-
-	for _, env := range os.Environ() {
-		parts := strings.SplitN(env, "=", 2)
-		if len(parts) == 2 {
-			envMap[parts[0]] = parts[1]
-		}
-	}
-
-	return envMap
-}
-
 // CopyFileContents copies contents of the given src file to the dst file
 func CopyFileContents(src, dst string) (err error) {
 	in, err := os.Open(src)