Import BLS to execution changes before Capella (#3892)

* Import BLS to execution changes before Capella

* Test for BLS to execution change HTTP API

* Pack BLS to execution changes in LIFO order

* Remove unused var

* Clippy

common/eth2/src/lib.rs

@@ -1042,6 +1042,24 @@ impl BeaconNodeHttpClient {
         Ok(())
     }
 
+    /// `POST beacon/pool/bls_to_execution_changes`
+    pub async fn post_beacon_pool_bls_to_execution_changes(
+        &self,
+        address_changes: &[SignedBlsToExecutionChange],
+    ) -> Result<(), Error> {
+        let mut path = self.eth_path(V1)?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("beacon")
+            .push("pool")
+            .push("bls_to_execution_changes");
+
+        self.post(path, &address_changes).await?;
+
+        Ok(())
+    }
+
     /// `GET beacon/deposit_snapshot`
     pub async fn get_deposit_snapshot(&self) -> Result<Option<types::DepositTreeSnapshot>, Error> {
         use ssz::Decode;