Expose default values in HamiltonNode

[elijahbenizzy]

Is your feature request related to a problem? Please describe.
We have optional_dependencies but its just a set. We don't expose it.

Describe the solution you'd like
We should have a default_values field or something like that. Dict[str, Any].

Describe alternatives you've considered
People can inspect the function -- see https://github.com/DAGWorks-Inc/hamilton/blob/main/hamilton/plugins/h_experiments/hook.py for instance.

Additional context
Feature request from OS. This is tricky as we don't capture it, but should be possible with a bit of rewiring.

[zilto]

relates to #495