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.swagger.json

@@ -4182,6 +4182,11 @@
           "type": "string",
           "format": "uint64",
           "description": "The confirmed SCID for a zero-conf channel."
+        },
+        "custom_channel_data": {
+          "type": "string",
+          "format": "byte",
+          "description": "The TLV encoded custom channel data records for this output, which might\nbe set for custom channels."
         }
       }
     },