routing: add source parameter to query routes

This commit allows execution of QueryRoutes from any source node.
Previously this was restricted to only the self node.

lnrpc/rpc.proto

@@ -1490,6 +1490,12 @@ message QueryRoutesRequest {
     A list of edges to ignore during path finding.
     */
     repeated EdgeLocator ignored_edges = 7;
+
+    /**
+    The source node where the request route should originated from. If empty,
+    self is assumed.
+    */
+    string source_pub_key = 8;
 }
 
 message EdgeLocator {

lnrpc/rpc.swagger.json

@@ -604,6 +604,13 @@
               "type": "string",
               "format": "byte"
             }
+          },
+          {
+            "name": "source_pub_key",
+            "description": "*\nThe source node where the request route should originated from. If empty,\nself is assumed.",
+            "in": "query",
+            "required": false,
+            "type": "string"
           }
         ],
         "tags": [