lnrpc+itest: return channel Memo for Pending channels

In a previous PR we added a Memo field for channels that could be
specified when opening a channel. This was a reference note-to-self
with no bearing on the functioning of the channel. In that PR, the
memo value was returned only through ListChannels. This commit builds
upon that PR by also returning the Memo field for channels returned by
PendingChannels RPC.

lnrpc/lightning.swagger.json

@@ -3028,6 +3028,10 @@
         "private": {
           "type": "boolean",
           "description": "Whether this channel is advertised to the network or not."
+        },
+        "memo": {
+          "type": "string",
+          "description": "An optional note-to-self to go along with the channel containing some\nuseful information. This is only ever stored locally and in no way\nimpacts the channel's operation."
         }
       }
     },