config: expose gossip.pinned-syncers for conf

The pinned syncer set is exposed as a comma-separated list of pubkeys.

sample-lnd.conf

@@ -1019,3 +1019,20 @@ litecoin.node=ltcd
 
 ; Specify the timeout to be used when opening the database.
 ; db.bolt.dbtimeout=60s
+
+[gossip]
+; Specify a set of pinned gossip syncers, which will always be actively syncing
+; whenever the corresponding peer is online. A pinned syncer does not count
+; towards the configured `numgraphsyncpeers` since pinned syncers are not
+; rotated. Configuring a pinned syncer does not ensure a persistent connection
+; to the target peer, they will only be pinned if the connection remains active
+; via some other mechanism, e.g. having an open channel.
+;
+; This feature is useful when trying to ensure that a node keeps its
+; routing table tightly synchronized with a set of remote peers, e.g. multiple
+; lightning nodes operated by the same service.
+;
+; Each value should be a hex-encoded pubkey of the pinned peer. Multiple pinned
+; peers can be specified by setting multiple flags/fields in the config.
+; gossip.pinned-syncers=pubkey1
+; gossip.pinned-syncers=pubkey2