lnrpc+rpcserver: add custom channel data for closed channels

This commit adds the custom channel data for closed channels which
represents the initial funding state as well as the final balances at
closing time.

lnrpc/lightning.proto

@@ -1758,6 +1758,10 @@ message ChannelCloseSummary {
 
     //  The confirmed SCID for a zero-conf channel.
     uint64 zero_conf_confirmed_scid = 15 [jstype = JS_STRING];
+
+    // The TLV encoded custom channel data records for this output, which might
+    // be set for custom channels.
+    bytes custom_channel_data = 16;
 }
 
 enum ResolutionType {