Add parent_block_number to payload SSE (#4053)

## Issue Addressed

In #4027 I forgot to add the `parent_block_number` to the payload attributes SSE.

## Proposed Changes

Compute the parent block number while computing the pre-payload attributes. Pass it on to the SSE stream.

## Additional Info

Not essential for v3.5.1 as I suspect most builders don't need the `parent_block_root`. I would like to use it for my dummy no-op builder however.

beacon_node/execution_layer/src/lib.rs

@@ -47,7 +47,7 @@ use types::{
 
 mod block_hash;
 mod engine_api;
-mod engines;
+pub mod engines;
 mod keccak;
 mod metrics;
 pub mod payload_cache;