MM-63285: Add property field methods to plugin API (#31035)

Co-authored-by: Claude <noreply@anthropic.com>

server/channels/app/app.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mattermost/mattermost/server/public/shared/httpservice"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/public/shared/timezones"
+	"github.com/mattermost/mattermost/server/v8/channels/app/properties"
 	"github.com/mattermost/mattermost/server/v8/einterfaces"
 	"github.com/mattermost/mattermost/server/v8/platform/services/imageproxy"
 	"github.com/mattermost/mattermost/server/v8/platform/services/searchengine"
@@ -154,6 +155,10 @@ func (a *App) SetServer(srv *Server) {
 	a.ch.srv = srv
 }
 
+func (a *App) PropertyService() *properties.PropertyService {
+	return a.Srv().propertyService
+}
+
 func (a *App) UpdateExpiredDNDStatuses() ([]*model.Status, error) {
 	return a.Srv().Store().Status().UpdateExpiredDNDStatuses()
 }