pdl: Complete the python backend generator

- Implement the missing serializer method.
- Implement an additional constructor for generated
  packets and stucts which uses default field initializers,
  vs. the default dataclass constructor which is stricter.
- Extend python_generator_tests.py with serializer tests.

Test: atest --host pdl_python_generator_test
Change-Id: I9cd93bb5227271acd58d5810369cca73fc1c6447