Add Merge support to web3signer validators (#3318)

## Issue Addressed

Web3signer validators can't produce post-Bellatrix blocks.

## Proposed Changes

Add support for Bellatrix to web3signer validators.

## Additional Info

I am running validators with this code on Ropsten, but it may be a while for them to get a proposal.

testing/web3signer_tests/src/lib.rs

@@ -612,6 +612,28 @@ mod tests {
             .await;
     }
 
+    /// Test all the Merge types.
+    async fn test_merge_types(network: &str, listen_port: u16) {
+        let network_config = Eth2NetworkConfig::constant(network).unwrap().unwrap();
+        let spec = &network_config.chain_spec::<E>().unwrap();
+        let merge_fork_slot = spec
+            .bellatrix_fork_epoch
+            .unwrap()
+            .start_slot(E::slots_per_epoch());
+
+        TestingRig::new(network, spec.clone(), listen_port)
+            .await
+            .assert_signatures_match("beacon_block_merge", |pubkey, validator_store| async move {
+                let mut merge_block = BeaconBlockMerge::empty(spec);
+                merge_block.slot = merge_fork_slot;
+                validator_store
+                    .sign_block(pubkey, BeaconBlock::Merge(merge_block), merge_fork_slot)
+                    .await
+                    .unwrap()
+            })
+            .await;
+    }
+
     #[tokio::test]
     async fn mainnet_base_types() {
         test_base_types("mainnet", 4242).await
@@ -631,4 +653,19 @@ mod tests {
     async fn prater_altair_types() {
         test_altair_types("prater", 4247).await
     }
+
+    #[tokio::test]
+    async fn ropsten_base_types() {
+        test_base_types("ropsten", 4250).await
+    }
+
+    #[tokio::test]
+    async fn ropsten_altair_types() {
+        test_altair_types("ropsten", 4251).await
+    }
+
+    #[tokio::test]
+    async fn ropsten_merge_types() {
+        test_merge_types("ropsten", 4252).await
+    }
 }

validator_client/src/signing_method/web3signer.rs

@@ -25,6 +25,7 @@ pub enum MessageType {
 pub enum ForkName {
     Phase0,
     Altair,
+    Bellatrix,
 }
 
 #[derive(Debug, PartialEq, Serialize)]
@@ -43,7 +44,10 @@ pub enum Web3SignerObject<'a, T: EthSpec, Payload: ExecPayload<T>> {
     Attestation(&'a AttestationData),
     BeaconBlock {
         version: ForkName,
-        block: &'a BeaconBlock<T, Payload>,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        block: Option<&'a BeaconBlock<T, Payload>>,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        block_header: Option<BeaconBlockHeader>,
     },
     #[allow(dead_code)]
     Deposit {
@@ -70,13 +74,23 @@ pub enum Web3SignerObject<'a, T: EthSpec, Payload: ExecPayload<T>> {
 
 impl<'a, T: EthSpec, Payload: ExecPayload<T>> Web3SignerObject<'a, T, Payload> {
     pub fn beacon_block(block: &'a BeaconBlock<T, Payload>) -> Result<Self, Error> {
-        let version = match block {
-            BeaconBlock::Base(_) => ForkName::Phase0,
-            BeaconBlock::Altair(_) => ForkName::Altair,
-            BeaconBlock::Merge(_) => return Err(Error::MergeForkNotSupported),
-        };
-
-        Ok(Web3SignerObject::BeaconBlock { version, block })
+        match block {
+            BeaconBlock::Base(_) => Ok(Web3SignerObject::BeaconBlock {
+                version: ForkName::Phase0,
+                block: Some(block),
+                block_header: None,
+            }),
+            BeaconBlock::Altair(_) => Ok(Web3SignerObject::BeaconBlock {
+                version: ForkName::Altair,
+                block: Some(block),
+                block_header: None,
+            }),
+            BeaconBlock::Merge(_) => Ok(Web3SignerObject::BeaconBlock {
+                version: ForkName::Bellatrix,
+                block: None,
+                block_header: Some(block.block_header()),
+            }),
+        }
     }
 
     pub fn message_type(&self) -> MessageType {