integrate light client into beacon node

[etan-status]

Adds a LightClient instance to the beacon node as preparation to
accelerate syncing in the future (optimistic sync).

• --light-client-enable turns on the feature
• --light-client-trusted-block-root configures block to start from

If no block root is configured, light client tracks DAG finalizedHead.

[github-actions]

Unit Test Results

     12 files  ±0     842 suites  ±0   56m 57s ⏱️ + 1m 4s
1 699 tests ±0  1 647 ✔️ ±0    52 💤 ±0  0 ❌ ±0 
9 893 runs  ±0  9 765 ✔️ ±0  128 💤 ±0  0 ❌ ±0 

Results for commit 24e77a9. ± Comparison against base commit f929980.

♻️ This comment has been updated with latest results.

[etan-status]

• add comment for values iterator purpose
• proc -> func for callback implementations
• len.uint -> lenu64 for length logging
• a .. (b - 1) -> a ..< b for conciseness
• queryed -> queried typo

[etan-status]

Note that this PR does not yet take into account GossipSub data. Will submit that separately as it's already quite noisy. GossipSub data can be used to keep store.optimistic_header updated.

[etan-status]

• Rebase to unstable (genesisValidatorsRoot -> genesis_validators_Root).

[etan-status]

• Reworked to use light client library from add light client (standalone) #3653
• Attach to gossip
• Sync from dag.finalizedHead if no light-client-trusted-block-root is configured

[tersec]

How does this Option/applyConfigDefault help?

nimbus-eth2/beacon_chain/nimbus_beacon_node.nim

Lines 1699 to 1709 in 43876fc

	template applyConfigDefault(field: untyped): untyped =
	if config.`field`.isNone:
	if not metadata.configDefaults.`field`.isZeroMemory:
	info "Applying network config default",
	eth2Network = config.eth2Network,
	`field` = metadata.configDefaults.`field`
	config.`field` = some metadata.configDefaults.`field`
	applyConfigDefault(lightClientEnable)
	applyConfigDefault(serveLightClientData)
	applyConfigDefault(importLightClientData)

show the mechanism, but it's not clear to me why this version (see also lightClientTrustedBlockRoot) is better than the more straightforward Option-free variation which sets some appropriate default and avoids the Option-.get() indirection.

[etan-status]

Had it at default-on for kiln/ropsten/prater, but you are right that now with the default off the straight-forward bool would also work. I guess when optimistic sync is fully implemented, that this would need to be changed to detect the case of "no user preference" vs "user selected off" and apply network specific default in first case. I can get rid of it once more and it can then be reintroduced later, or we can just keep it as is, up to you.

[tersec]

Alright, keep as is.

[etan-status]

During the Ropsten change from Altair -> Bellatrix, noticed in the logs that the LC optimistic update for the first slot was not picked up on Gossip, due to the subscription on old net being dropped immediately after bellatrix. Light client uses attested_header to derive fork digest, but sends the message based on a future signature_slot, so the topics need to be available some longer. Will change it so that Altair light client subscriptions stay around for the first bellatrix epoch as well, to avoid missing the first slot.