[management] check and log on new management version (#4029)

This PR enhances the version checker to send a custom User-Agent header when polling for updates, and configures both the management CLI and client UI to use distinct agents. 

- NewUpdate now takes an `httpAgent` string to set the User-Agent header.
- `fetchVersion` builds a custom HTTP request (instead of `http.Get`) and sets the User-Agent.
- Management CLI and client UI now pass `"nb/management"` and `"nb/client-ui"` respectively to NewUpdate.
- Tests updated to supply an `httpAgent` constant.
- Logs if there is a new version available for management

version/update_test.go

@@ -9,6 +9,8 @@ import (
 	"time"
 )
 
+const httpAgent = "pkg/test"
+
 func TestNewUpdate(t *testing.T) {
 	version = "1.0.0"
 	svr := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -21,7 +23,7 @@ func TestNewUpdate(t *testing.T) {
 	wg.Add(1)
 
 	onUpdate := false
-	u := NewUpdate()
+	u := NewUpdate(httpAgent)
 	defer u.StopWatch()
 	u.SetOnUpdateListener(func() {
 		onUpdate = true
@@ -46,7 +48,7 @@ func TestDoNotUpdate(t *testing.T) {
 	wg.Add(1)
 
 	onUpdate := false
-	u := NewUpdate()
+	u := NewUpdate(httpAgent)
 	defer u.StopWatch()
 	u.SetOnUpdateListener(func() {
 		onUpdate = true
@@ -71,7 +73,7 @@ func TestDaemonUpdate(t *testing.T) {
 	wg.Add(1)
 
 	onUpdate := false
-	u := NewUpdate()
+	u := NewUpdate(httpAgent)
 	defer u.StopWatch()
 	u.SetOnUpdateListener(func() {
 		onUpdate = true