MM-63327 Config setting for ServiceSettings.FrameAncestors (#30409)

* Add Embedding page to system console, with single setting for Frame Ancestors

server/public/model/config.go

@@ -440,6 +440,7 @@ type ServiceSettings struct {
 	MaximumURLLength                                  *int    `access:"environment_file_storage,write_restrictable,cloud_restrictable"`
 	ScheduledPosts                                    *bool   `access:"site_posts"`
 	EnableWebHubChannelIteration                      *bool   `access:"write_restrictable,cloud_restrictable"` // telemetry: none
+	FrameAncestors                                    *string `access:"write_restrictable,cloud_restrictable"` // telemetry: none
 }
 
 var MattermostGiphySdkKey string
@@ -967,6 +968,10 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	if s.EnableWebHubChannelIteration == nil {
 		s.EnableWebHubChannelIteration = NewPointer(false)
 	}
+
+	if s.FrameAncestors == nil {
+		s.FrameAncestors = NewPointer("")
+	}
 }
 
 type CacheSettings struct {