Deduplicate block root computation (#3590)

## Issue Addressed

NA

## Proposed Changes

This PR removes duplicated block root computation.

Computing the `SignedBeaconBlock::canonical_root` has become more expensive since the merge as we need to compute the merke root of each transaction inside an `ExecutionPayload`.

Computing the root for [a mainnet block](https://beaconcha.in/slot/4704236) is taking ~10ms on my i7-8700K CPU @ 3.70GHz (no sha extensions). Given that our median seen-to-imported time for blocks is presently 300-400ms, removing a few duplicated block roots (~30ms) could represent an easy 10% improvement. When we consider that the seen-to-imported times include operations *after* the block has been placed in the early attester cache, we could expect the 30ms to be more significant WRT our seen-to-attestable times.

## Additional Info

NA

beacon_node/http_api/tests/interactive_tests.rs

@@ -67,7 +67,10 @@ pub async fn fork_choice_before_proposal() {
 
     let state_a = harness.get_current_state();
     let (block_b, state_b) = harness.make_block(state_a.clone(), slot_b).await;
-    let block_root_b = harness.process_block(slot_b, block_b).await.unwrap();
+    let block_root_b = harness
+        .process_block(slot_b, block_b.canonical_root(), block_b)
+        .await
+        .unwrap();
 
     // Create attestations to B but keep them in reserve until after C has been processed.
     let attestations_b = harness.make_attestations(
@@ -80,7 +83,7 @@ pub async fn fork_choice_before_proposal() {
 
     let (block_c, state_c) = harness.make_block(state_a, slot_c).await;
     let block_root_c = harness
-        .process_block(slot_c, block_c.clone())
+        .process_block(slot_c, block_c.canonical_root(), block_c.clone())
         .await
         .unwrap();