Minimal valid SignedBeaconBlock construction

[mpaulucci]

This is part of #801

The idea it to create the simplest valid block that can be propagated through the network.

So the scope for this block is:

• Compute starting state with get_head.
• Compute the parent root as get_proposer_head(store, head_root, slot)
• Set the block body with:
  • Compute randao_reveal
  • Copy eth1_data from previous block
  • Set all operations as empty lists (proposer slashing, attester slashing, pending attestations, etc..)
  • Call the execution layer with get_execution_payload. Add KZG commitments returned from the execution layer to the blob
  • Calculate sync_aggregate as if no sync commitee contributions had been made
• Compute compute_new_state_root. Apply state transition to previous state and calculate the root to add it to the block.
• Sign the block.

[MegaRedHand]

The proposal should be done in

lambda_ethereum_consensus/lib/lambda_ethereum_consensus/validator/validator.ex

Lines 398 to 400 in 0e2858a

	defp propose(_state) do
	# TODO: implement block proposal
	end