Add _asdict so dut ID can be serialised

Author: jonny12375

openhtf/core/dut_id.py

@@ -13,3 +13,12 @@ class DutIdentifier:
     @property
     def test_id(self) -> str:
         return self.halter_serial_number
+
+    def _asdict(self) -> dict:
+        return {
+            "halter_serial_number": self.halter_serial_number,
+            "manufacturer_serial_number": self.manufacturer_serial_number,
+            "mac_address": self.mac_address,
+            "part_number": self.part_number,
+            "additional": self.additional
+        }