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/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),),