api: add endpoint serving the project .gns3 topology file

GET /v3/projects/{project_id}/gns3file returns the raw .gns3 file as
application/json so the WebUI can render topology thumbnails without
opening the project. Adds a public Project.topology_file property.

gns3server/controller/project.py

@@ -1685,6 +1685,14 @@ class Project:
     def _topology_file(self):
         return os.path.join(self.path, self._filename)
 
+    @property
+    def topology_file(self):
+        """
+        Absolute path of the .gns3 topology file
+        """
+
+        return self._topology_file()
+
     @locking
     async def open(self, auto_start=True):
         """