multi: add experimental endorsement feature bit and disable option

sample-lnd.conf

@@ -1399,6 +1399,9 @@
 ; Set to disable blinded route forwarding.
 ; protocol.no-route-blinding=false
 
+; Set to disable experimental endorsement signaling.
+; protocol.no-experimental-endorsement=false
+
 ; Set to handle messages of a particular type that falls outside of the
 ; custom message number range (i.e. 513 is onion messages). Note that you can
 ; set this option as many times as you want to support more than one custom