lnrpc: add additional REST binding for QueryRoutes

Lists with non-primitive members aren't supported in the query string of
a GET request with the current version of the grpc-gateway library. To
allow route_hints to be set through REST, we also offer a POST endpoint
for that call where the encoding of the request parameter can be
specified as JSON.

lnrpc/lightning.yaml

@@ -111,6 +111,9 @@ http:
       get: "/v1/graph/node/{pub_key}"
     - selector: lnrpc.Lightning.QueryRoutes
       get: "/v1/graph/routes/{pub_key}/{amt}"
+      additional_bindings:
+        - post: "/v1/graph/routes/{pub_key}/{amt}"
+          body: "*"
     - selector: lnrpc.Lightning.GetNetworkInfo
       get: "/v1/graph/info"
     - selector: lnrpc.Lightning.StopDaemon