Refactor `BlockProposalContents` for Electra

[michaelsproul]

There's a TODO here about refactoring BlockProposalContents to make it more explicit when the ExecutionRequests are set or not.

lighthouse/beacon_node/execution_layer/src/lib.rs

Lines 200 to 214 in 0728140

	pub enum BlockProposalContents<E: EthSpec, Payload: AbstractExecPayload<E>> {
	Payload {
	payload: Payload,
	block_value: Uint256,
	},
	PayloadAndBlobs {
	payload: Payload,
	block_value: Uint256,
	kzg_commitments: KzgCommitments<E>,
	/// `None` for blinded `PayloadAndBlobs`.
	blobs_and_proofs: Option<(BlobsList<E>, KzgProofs<E>)>,
	// TODO(electra): this should probably be a separate variant/superstruct
	requests: Option<ExecutionRequests<E>>,
	},
	}

I agree we should refactor this so that it is more strongly typed and we avoid the combinatorial blowup of having multiple options.

See similar PR here:

• Block availability data enum #6866

[michaelsproul]

There is no urgency for this refactor, hence it doesn't need to be in v7.x

[varun-doshi]

@michaelsproul raised a PR for this. Do let me know if this was the intended direction