serve libp2p protocol for light client sync

[etan-status]

This extends the --serve-light-client-data launch option to serve
locally collected light client data via libp2p.
Backfill of historic best LightClientUpdate is not yet implemented.
See ethereum/consensus-specs#2802

[github-actions]

Unit Test Results

     12 files  ±0     834 suites  ±0   54m 20s ⏱️ +13s
1 686 tests ±0  1 638 ✔️ ±0    48 💤 ±0  0 ❌ ±0 
9 825 runs  ±0  9 713 ✔️ ±0  112 💤 ±0  0 ❌ ±0 

Results for commit 178ba5d. ± Comparison against base commit e7d017b.

♻️ This comment has been updated with latest results.

[etan-status]

Rebased to unstable, updated spec URLs for 1.1.9, and re-phrased epochBoundarySlot helper documentation.

[etan-status]

Fixed time delay of broadcasting OptimisticLightClientUpdate, it was combined with attestations and sync committee messages, but those may fire earlier than 1/3 of slot. OptimisticLightClientUpdate is always sent at 1/3.

Extended sendBeaconBlock REST/JSON-RPC API to send the OptimisticLightClientUpdate as well.

Avoid "No optimistic light client update for proposed block" log when proposing pre-Altair block.

[etan-status]

• Rebased to unstable.
• Update to latest proposed libp2p spec (allowForceUpdate parameter).
  Define libp2p protocol for light client sync ethereum/consensus-specs#2802
• Verify update fork_version against local RuntimeConfig during light client sync.

[etan-status]

• Updated sync_protocol for latest style (early return for step == 0 / count == 0 case, and trace before throwing exception).

[etan-status]

Thanks to @arnetheduck for the detailed review!

• Document GeneralizedIndex constants in altair.nim.

• Add new option --import-light-client-data=on-demand to compute historic light client data on demand instead of at startup. May lead to missed validator duties.

• Replace --import-historic-light-client-data=false with --import-light-client-data=only-new.

• Replace --import-historic-light-client-data=true with --import-light-client-data=full.

• Harden initBestLightClientUpdateForPeriod / initLightClientBootstrapForPeriod for untrusted period input.

• Move light client database initialization from blockchain_dag.nim into blockchain_dag_light_client.nim.

• Move light client specifics from block_pools_types.nim into block_pools_types_light_client.nim.

• Wrap light client database objects into LightClientDatabase and renamed field names to be a bit shorter.

• Re-use StateCache when operating on multiple states in loops.

• Avoid caching of unnecessary data in edge cases during light client database initialization.

• Use version 0 for libp2p protocol IDs to signify this being for a pre-release specification.

• Move handleProposal changes to call side, and renamed didPropose to didSubmitBlock.

[etan-status]

• Update test instructions in PR description for new command line options.

[etan-status]

• Tag new libp2p req/rsp endpoints to be served only when the --serve-light-client-data is specified. See eth2_network.nim changes.

[etan-status]

• Rebase to unstable - this leaves only the Req/Resp changes in this PR :)

[etan-status]

• Rebase to unstable (Option -> Opt for LightClientBoostrap endpoint)

[etan-status]

• Remove step parameter from BestLightClientUpdatesByRange message
• Replace unreachable serveLightClientData checks with asserts