auth-app: Allow to set label via API

Instead of using the same hardcoded label for all app tokens, allow to
specify one on the API.
When an app token is generate via the impersonation feature we add the
string ` (Impersonation)` to the label.

services/auth-app/pkg/service/service.go

@@ -99,7 +99,10 @@ func (a *AuthAppService) HandleCreate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	label := "Generated via API"
+	label := q.Get("label")
+	if label == "" {
+		label = "Generated via API"
+	}
 
 	// Impersonated request
 	userID, userName := q.Get("userID"), q.Get("userName")
@@ -131,7 +134,7 @@ func (a *AuthAppService) HandleCreate(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		label = "Generated via Impersonation API"
+		label = label + " (Impersonation)"
 	}
 
 	scopes, err := scope.AddOwnerScope(map[string]*authpb.Scope{})