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>

consensus/state_processing/src/all_caches.rs

@@ -1,9 +1,7 @@
 use crate::common::update_progressive_balances_cache::initialize_progressive_balances_cache;
 use crate::epoch_cache::initialize_epoch_cache;
 use tracing::instrument;
-use types::{
-    BeaconState, ChainSpec, EpochCacheError, EthSpec, FixedBytesExtended, Hash256, RelativeEpoch,
-};
+use types::{BeaconState, ChainSpec, EpochCacheError, EthSpec, Hash256, RelativeEpoch};
 
 /// Mixin trait for the beacon state that provides operations on *all* caches.
 ///
@@ -34,8 +32,7 @@ impl<E: EthSpec> AllCaches for BeaconState<E> {
 
     fn all_caches_built(&self) -> bool {
         let current_epoch = self.current_epoch();
-        let Ok(epoch_cache_decision_block_root) =
-            self.proposer_shuffling_decision_root(Hash256::zero())
+        let Ok(epoch_cache_decision_block_root) = self.epoch_cache_decision_root(Hash256::ZERO)
         else {
             return false;
         };