Compute proposer shuffling only once in gossip verification (#7304)

When we perform data column gossip verification, we sometimes see multiple proposer shuffling cache miss simultaneously and this results in multiple threads computing the shuffling cache and potentially slows down the gossip verification.

Proposal here is to use a `OnceCell` for each shuffling key to make sure it's only computed once. I have only implemented this in data column verification as a PoC, but this can also be applied to blob and block verification

Related issues:
- https://github.com/sigp/lighthouse/issues/4447
- https://github.com/sigp/lighthouse/issues/7203

beacon_node/beacon_chain/Cargo.toml

@@ -47,6 +47,7 @@ logging = { workspace = true }
 lru = { workspace = true }
 merkle_proof = { workspace = true }
 metrics = { workspace = true }
+once_cell = { workspace = true }
 oneshot_broadcast = { path = "../../common/oneshot_broadcast/" }
 operation_pool = { workspace = true }
 parking_lot = { workspace = true }