feat: add Header::num_hash_slow (alloy-rs#1357)

crates/consensus/src/header.rs

@@ -345,10 +345,19 @@ impl Header {
     }
 
     /// Returns the parent block's number and hash
+    ///
+    /// Note: for the genesis block the parent number is 0 and the parent hash is the zero hash.
     pub const fn parent_num_hash(&self) -> BlockNumHash {
         BlockNumHash { number: self.number.saturating_sub(1), hash: self.parent_hash }
     }
 
+    /// Returns the block's number and hash.
+    ///
+    /// Note: this hashes the header.
+    pub fn num_hash_slow(&self) -> BlockNumHash {
+        BlockNumHash { number: self.number, hash: self.hash_slow() }
+    }
+
     /// Checks if the block's difficulty is set to zero, indicating a Proof-of-Stake header.
     ///
     /// This function is linked to EIP-3675, proposing the consensus upgrade to Proof-of-Stake: