## Issue Addressed
- Resolves#2502
## Proposed Changes
Adds tree-hash caching (THC 🍁) for `state.inactivity_scores`, as per #2502.
Since the `inactivity_scores` field is introduced during Altair, the cache must be optional (i.e., not present pre-Altair). The mechanism for optional caches was already implemented via the `ParticipationTreeHashCache`, albeit not quite generically enough. To this end, I made the `ParticipationTreeHashCache` more generic and renamed it to `OptionalTreeHashCache`. This made the code a little more verbose around the previous/current epoch participation fields, but overall less verbose when the needs of `inactivity_scores` are considered.
All changes to `ParticipationTreeHashCache` should be *non-substantial*.
## Additional Info
NA
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