Mandatory fee recipient

[michaelsproul]

Description

We should consider making the fee_recipient mandatory, to avoid blocks going to junk addresses

See: https://twitter.com/Butta_eth/status/1767632279680024759

Lighthouse doesn't actually default to 0x00, but there have still been some block rewards sent to our default 0x01, although not for >50 days at time of writing.

lighthouse/beacon_node/execution_layer/src/lib.rs

Lines 84 to 85 in 2a3c709

	const DEFAULT_SUGGESTED_FEE_RECIPIENT: [u8; 20] =
	[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];

Steps to resolve

The hard part is deciding how and where to make it mandatory. There are many places the fee recipient can be set:

1. Fallback fee recipient in the beacon node. Should this be mandatory? Probably not?
2. Fallback fee recipient in the validator client. Probably the best candidate for being made mandatory.
3. Per-validator fee recipient from validator_definitions.yaml. We could make it mandatory that some fee recipient is set for every validator, either all validators are configured with an individual fee recipient, or the fallback recipient is set. This could have better UX than making the fallback mandatory, as many users run validators on behalf of other entities and don't want to set a fallback address.