[Explore] Investigate changes needed for the merge

[jframe]

Investigate what impact the merge has on Web3Signer and what changes if any are needed.

Already identified

• Either need to change API to include Eth1 block data or use the block body root see Only require block_body_root instead of full block body #437

Acceptance Criteria

• Identify what changes are needed to Web3Signer to support the merge
• Document how the changes affect Web3Signer
• Identify when this needs to be done by
• What testnets are available

[jframe]

Changes needed for Web3signer. For Bellatrix either the Beacon block body needs to be updated to include the execution payload or the beacon block body is replaced with the beacon block body root. Preferred approach is to use the beacon body root #437 as this will reduce that amount data is sent, parsed and simplify the API.