Check for terminal block during proposer preparation

[paulhauner]

Description

The merge transition block is the first post-Bellatrix block to include a non-default ExecutionPayload. The execution_payload.parent_hash value is computed here:

lighthouse/beacon_node/beacon_chain/src/execution_payload.rs

Lines 264 to 267 in c3a793f

	let terminal_pow_block_hash = execution_layer
	.get_terminal_pow_block_hash(spec)
	.await
	.map_err(BlockProductionError::TerminalPoWBlockLookupFailed)?;

According to the validator guide, we're supposed to call forkchoiceUpdated (fcU) before producing that block with the head_block_hash set to the value returned from ExecutionLayer::get_terminal_pow_block_hash.

In #3043, I've added the functionality to give the EL a "heads-up" for payload production via a PayloadAttributes value in a fcU call. However, I haven't handled the edge-case of the transition block.

To handle the edge case, I think we need to call ExecutionLayer::get_terminal_pow_block_hash in BeaconChain::prepare_proposer_async whenever we're post-Bellatrix but the head block does not have execution enabled. If we get a terminal pow block hash, then we should use that value as the head_block_root rather than the value that's presently on the head block.

We might actually be able to do it a bit cleaner, where the call to get_terminal_pow_block_hash is done in BeaconChain::update_execution_engine_forkchoice_async. I'm not sure what the best implementation is, yet.

Importantly, with our current implementation, we'll just fail to give advance notice of the terminal block. However, we'll still produce a block since we'll do a valid fcU call immediately before we try to produce the transition payload.

[ethDreamer]

I'll take this one

[ethDreamer]

Diagram of execution flow for reference