Add abstractions for filtering backward model proposals

[kmaziarz]

One important missing feature of syntheseus is support for filtering backward model proposals during search, for example by using a forward model or a feasibility model. This PR proposes some initial abstractions to do that, notably ReactionFilterModel to encapsulate all logic that filters proposed reaction steps. Given that filter and scoring models would also benefit from caching, I first extracted the caching-related logic from ReactionModel into a shared base class.

After we agree on these abstractions, the next step will be to add hooks into the search CLI to then enable basic forms of filtering during search, and log more statistics related to filtering itself (e.g. acceptance rates and fraction of time spent on filter model vs backward).

This PR is split into logical commits for easier reviewing.

[AustinT]

@kmaziarz sorry for silence from me- last week was quite busy. I don't have time to look in detail now but the feature makes sense, thanks for adding!