Fix bugs in proposer calculation post-Fulu (#8101)

As identified by a researcher during the Fusaka security competition, we were computing the proposer index incorrectly in some places by computing without lookahead.


  - [x] Add "low level" checks to computation functions in `consensus/types` to ensure they error cleanly
- [x] Re-work the determination of proposer shuffling decision roots, which are now fork aware.
- [x] Re-work and simplify the beacon proposer cache to be fork-aware.
- [x] Optimise `with_proposer_cache` to use `OnceCell`.
- [x] All tests passing.
- [x] Resolve all remaining `FIXME(sproul)`s.
- [x] Unit tests for `ProtoBlock::proposer_shuffling_root_for_child_block`.
- [x] End-to-end regression test.
- [x] Test on pre-Fulu network.
- [x] Test on post-Fulu network.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/errors.rs

@@ -230,6 +230,23 @@ pub enum BeaconChainError {
         columns_found: usize,
     },
     FailedToReconstructBlobs(String),
+    ProposerCacheIncorrectState {
+        state_decision_block_root: Hash256,
+        requested_decision_block_root: Hash256,
+    },
+    ProposerCacheAccessorFailure {
+        decision_block_root: Hash256,
+        proposal_epoch: Epoch,
+    },
+    ProposerCacheOutOfBounds {
+        slot: Slot,
+        epoch: Epoch,
+    },
+    ProposerCacheWrongEpoch {
+        request_epoch: Epoch,
+        cache_epoch: Epoch,
+    },
+    SkipProposerPreparation,
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);