Feature request: `with-dynamic-cost` support

[sklam]

with-dynamic-cost will be very useful for numba's future.

There's an example in the end of https://egraphs-good.github.io/egglog-tutorial/05-cost-model-and-extraction.html

[saulshanabrook]

Sounds good! Adding support for set-cost will be relatively easy and is in the next thing I will work on once we have a way to programmatically retrieve the results (egraphs-good/egglog#653).

Creating totally custom cost models in Python requires a bit more thought, because it requires exposing Values to Python to some degree, which so far I have managed to avoid:

https://github.com/egraphs-good/egglog-experimental/blob/9194ba8cec4be2e94eddd0595a355a5a57480120/src/set_cost.rs#L176-L208

It should be doable and is also required for custom schedulers, but just requires some more design.