routing+lnrpc: extend BuildRoute to accept raw payAddr

In this commit, we extend the `BuildRoute` method and RPC on the router
sub-server to accept a raw payment address which will be included as
part of an MPP payload for the finla hop. This change actually also
allows users to craft their own MPP paths using BuildRoute+SendToRoute.
Our primary goal however, was to fix some broken itests since we now
require the payAddr to be present for ALL payments other than key send
payments.

lnrpc/routerrpc/router.swagger.json

@@ -832,6 +832,11 @@
             "format": "byte"
           },
           "description": "A list of hops that defines the route. This does not include the source hop\npubkey."
+        },
+        "payment_addr": {
+          "type": "string",
+          "format": "byte",
+          "description": "An optional payment addr to be included within the last hop of the route."
         }
       }
     },