tailcfg: add exit destination for network flow logs node attribute (#11698)

Updates tailscale/corp#18625

Signed-off-by: Claire Wang <[email protected]>

tailcfg/tailcfg.go

@@ -2231,6 +2231,9 @@ const (
 
 	// NodeAttrDisableWebClient disables using the web client.
 	NodeAttrDisableWebClient NodeCapability = "disable-web-client"
+
+	// NodeAttrExitDstNetworkFlowLog enables exit node destinations in network flow logs.
+	NodeAttrExitDstNetworkFlowLog NodeCapability = "exit-dst-network-flow-log"
 )
 
 // SetDNSRequest is a request to add a DNS record.