Make ExecutionBlock::total_difficulty Optional (#7050)

This change makes the `total_difficulty` field in `ExecutionBlock` an `Option<Uint256>` since newer clients are no longer including the `totalDifficulty` field.

I think this will fix https://github.com/sigp/lighthouse/issues/6937 but I was actually more focused on the builder registration case described below.

In our [builder-playground](https://github.com/flashbots/builder-playground) we setup a local devnet using lighthouse, reth, and mev-boost-relay.  After upgrading to reth 1.2.0 and lighthouse v7.0.0.beta.0 for Pectra, we noticed that the validator registration process was _sometimes_ failing with:

```
Feb 25 23:35:25.038 ERRO Unable to publish proposer preparation to all beacon nodes, error: Some endpoints failed, num_failed: 1 http://localhost:3500/ => RequestFailed(ServerMessage(ErrorMessage { code: 400, message: "BAD_REQUEST: error updating proposer preparations: ForkchoiceUpdate(EngineError(Api { error: Json(Error(\"missing field `totalDifficulty`\", line: 0, column: 0)) }))", stacktraces: [] })), service: preparation
Feb 25 23:35:25.099 WARN Unable to publish validator registrations to the builder network, error: Some endpoints failed, num_failed: 1 http://localhost:3500/ => RequestFailed(ServerMessage(ErrorMessage { code: 400, message: "BAD_REQUEST: error updating proposer preparations: ForkchoiceUpdate(EngineError(Api { error: Json(Error(\"missing field `totalDifficulty`\", line: 0, column: 0)) }))", stacktraces: [] })), service: preparation
```

What was even more confusing, was that it was sometimes working, which actually led to a wild goose chase thinking it was a networking issue.  However, when tracing through the LH code, I came across this comment:

70194dfc6a/beacon_node/beacon_chain/src/beacon_chain.rs (L6048-L6049)

This explained why it sometimes worked, in our playground we run lighthouse with `--prepare-payload-lookahead 8000` thus there was always a 4-second window where the call wasn't made.

But, if the call was made, then this code would 100% fail with updated reth:

https://github.com/sigp/lighthouse/blob/unstable/beacon_node/execution_layer/src/lib.rs#L1688-L1692

Which would then mapped to a `Error::ForkchoiceUpdate` in `update_execution_engine_forkchoice`.


  Anyways, the fix was to make `total_difficulty` Optional, and then to update any code paths where it was used.  In doing so, I assume that if the EL doesn't include total difficulty then the chain is already post-merge.

beacon_node/beacon_chain/src/bellatrix_readiness.rs

@@ -171,7 +171,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 return BellatrixReadiness::NotSynced;
             }
             let params = MergeConfig::from_chainspec(&self.spec);
-            let current_difficulty = el.get_current_difficulty().await.ok();
+            let current_difficulty = el.get_current_difficulty().await.ok().flatten();
             BellatrixReadiness::Ready {
                 config: params,
                 current_difficulty,

beacon_node/execution_layer/src/engine_api.rs

@@ -142,11 +142,18 @@ pub struct ExecutionBlock {
     pub block_number: u64,
 
     pub parent_hash: ExecutionBlockHash,
-    pub total_difficulty: Uint256,
+    pub total_difficulty: Option<Uint256>,
     #[serde(with = "serde_utils::u64_hex_be")]
     pub timestamp: u64,
 }
 
+impl ExecutionBlock {
+    pub fn terminal_total_difficulty_reached(&self, terminal_total_difficulty: Uint256) -> bool {
+        self.total_difficulty
+            .is_none_or(|td| td >= terminal_total_difficulty)
+    }
+}
+
 #[superstruct(
     variants(V1, V2, V3),
     variant_attributes(derive(Clone, Debug, Eq, Hash, PartialEq),),

beacon_node/execution_layer/src/lib.rs

@@ -621,7 +621,7 @@ impl<E: EthSpec> ExecutionLayer<E> {
     }
 
     /// Get the current difficulty of the PoW chain.
-    pub async fn get_current_difficulty(&self) -> Result<Uint256, ApiError> {
+    pub async fn get_current_difficulty(&self) -> Result<Option<Uint256>, ApiError> {
         let block = self
             .engine()
             .api
@@ -1680,7 +1680,8 @@ impl<E: EthSpec> ExecutionLayer<E> {
         self.execution_blocks().await.put(block.block_hash, block);
 
         loop {
-            let block_reached_ttd = block.total_difficulty >= spec.terminal_total_difficulty;
+            let block_reached_ttd =
+                block.terminal_total_difficulty_reached(spec.terminal_total_difficulty);
             if block_reached_ttd {
                 if block.parent_hash == ExecutionBlockHash::zero() {
                     return Ok(Some(block));
@@ -1689,7 +1690,8 @@ impl<E: EthSpec> ExecutionLayer<E> {
                     .get_pow_block(engine, block.parent_hash)
                     .await?
                     .ok_or(ApiError::ExecutionBlockNotFound(block.parent_hash))?;
-                let parent_reached_ttd = parent.total_difficulty >= spec.terminal_total_difficulty;
+                let parent_reached_ttd =
+                    parent.terminal_total_difficulty_reached(spec.terminal_total_difficulty);
 
                 if block_reached_ttd && !parent_reached_ttd {
                     return Ok(Some(block));
@@ -1765,9 +1767,11 @@ impl<E: EthSpec> ExecutionLayer<E> {
         parent: ExecutionBlock,
         spec: &ChainSpec,
     ) -> bool {
-        let is_total_difficulty_reached = block.total_difficulty >= spec.terminal_total_difficulty;
-        let is_parent_total_difficulty_valid =
-            parent.total_difficulty < spec.terminal_total_difficulty;
+        let is_total_difficulty_reached =
+            block.terminal_total_difficulty_reached(spec.terminal_total_difficulty);
+        let is_parent_total_difficulty_valid = parent
+            .total_difficulty
+            .is_some_and(|td| td < spec.terminal_total_difficulty);
         is_total_difficulty_reached && is_parent_total_difficulty_valid
     }
 

beacon_node/execution_layer/src/test_utils/execution_block_generator.rs

@@ -84,14 +84,14 @@ impl<E: EthSpec> Block<E> {
                 block_hash: block.block_hash,
                 block_number: block.block_number,
                 parent_hash: block.parent_hash,
-                total_difficulty: block.total_difficulty,
+                total_difficulty: Some(block.total_difficulty),
                 timestamp: block.timestamp,
             },
             Block::PoS(payload) => ExecutionBlock {
                 block_hash: payload.block_hash(),
                 block_number: payload.block_number(),
                 parent_hash: payload.parent_hash(),
-                total_difficulty,
+                total_difficulty: Some(total_difficulty),
                 timestamp: payload.timestamp(),
             },
         }