Change default callback_filter in update_metadata

[esoteric-ephemera]

Changing the default value of callback_filter in the update_metadata function of both Job and Flow from a lambda to None. Having its default value be a lamdba function means that it cannot be round-trip de-/serialized:

from monty.json import MontyEncoder, MontyDecoder

f = lambda _ : True
encoded = MontyEncoder().encode(f)
print(encoded)
>>> '{"@module": "__main__", "@callable": "<lambda>", "@bound": null}'
decoded = MontyDecoder().decode(encoded)
print(decoded)
>>> {'@module': '__main__', '@callable': '<lambda>', '@bound': None}

This causes issues in fireworks with the newest release of jobflow. Linking relevant discussion here and tagging @janosh and @utf for thoughts

Because lambda's aren't pickle-able without something like dill, writing a callback_filter as an importable function might be the only option for fireworks / jobflow users

[janosh]

Looks good 👍 thank you for fixing!

[utf]

Thanks @esoteric-ephemera