Implement light client syncing

[zah]

To be broken down into smaller steps that will be tracked here as an epic.

[etan-status]

References:

• EF sync protocol spec (applying LightClientUpdate to LightClientStore)
  https://github.com/ethereum/consensus-specs/blob/v1.1.0/specs/altair/sync-protocol.md
• EF test suite (applying LightClientUpdate to LightClientStore)
  https://github.com/ethereum/consensus-specs/blob/v1.1.0/tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py
• Lodestar light client demo (requires some tweaks to run)
  https://github.com/ChainSafe/eth2-light-client-demo/tree/dapplion/update-to-prater
• Lodestar REST API (API and light client implementation is Apache 2, other components may be GPL)
  https://github.com/ChainSafe/lodestar/blob/v0.31.0/packages/api/src/routes/lightclient.ts
• C# light client demo
  https://github.com/uink45/Beacon-Chain-Light-Client-Prototype
• EF merkle multiproof spec
  https://github.com/ethereum/consensus-specs/blob/v1.1.3/ssz/merkle-proofs.md#merkle-multiproofs
• Lodestar merkle multiproof spec (different ordering than EF, plus metadata)
  https://github.com/protolambda/eth-merkle-trees/blob/master/tree_offsets.md
• Lodestar discord (ETH2 -> #light-client)
  https://discord.gg/b8WpNbPtqX
• Nimbus discord (DEV -> #light-client)
  https://discord.gg/xEYQ7RPS6k
• EF discord (CONSENSUS R&D -> #light-clients)
  https://discord.gg/FwhpbfwXUt
• Telegram (beacon-chain-light-client-implementers)
• [WIP] Light client protocol + Req/Resp interaction ethereum/consensus-specs#2267
• Simplify sync protocol and update to calculate optimistic heads ethereum/consensus-specs#2746
• (WIP) Light client p2p interface ethereum/consensus-specs#2786
• Fix sync protocol next_sync_committee_branch verification and remove active header ethereum/consensus-specs#2829
• Light-client (WORK IN PROGRESS) jinfwhuang/prysm#5

Specs:

1. Allow light client to verify signatures at period boundary ethereum/consensus-specs#2805
2. Remove fork_version from LightClientUpdate ethereum/consensus-specs#2916
3. Allow LightClientUpdate with genesis finality ethereum/consensus-specs#2924
4. Allow passing redundant next_sync_committee ethereum/consensus-specs#2927
5. Cleanup light client tests ethereum/consensus-specs#2929
6. Root next_sync_committee in attested_header ethereum/consensus-specs#2932
7. Add LightClientBootstrap ethereum/consensus-specs#2935
8. Add functions for deriving light client data ethereum/consensus-specs#2938
9. Add LightClient(Finality|Optimistic)Update ethereum/consensus-specs#2944
10. Manually trigger LightClientStore force updates ethereum/consensus-specs#2947
11. Describe light client sync process ethereum/consensus-specs#2951
12. Define libp2p protocol for light client sync ethereum/consensus-specs#2802

Implementation progress:

• Light client bootstrap (interfacing with checkpoint sync)
  allow trusted node sync based on LC trusted block root #4736
• Drive EL from LC
  allow driving EL with LC #3865
• Persisting light client data on server
  merge LC db into main BN db #3832
  persist LC data across restarts #3823
• Optimistic sync
  keep track of latest blocks for optimistic sync #3715
  • Integrate light client into beacon node
    integrate light client into beacon node #3557
• Light client mode
  add light client (standalone) #3653
• Enable light client data serving
  update to latest light client libp2p protocol #3623
  serve light client data on prater by default #3559
  • nimbus.prater: enable --*-light-client-data infra-nimbus#91
  • nimbus.kiln: enable --*-light-client-data infra-nimbus#89
    • nimbus: mark branches supporting BN light client server infra-nimbus#90
      • add beacon_node_light_client_data_* variables infra-role-beacon-node-linux#7
      • add beacon_node_light_client_data_* variables infra-role-beacon-node-macos#3
      • add beacon_node_light_client_data_* variables infra-role-beacon-node-windows#2
• Light client server
  serve libp2p protocol for light client sync #3341
  • Gossip broadcasts
    broadcast optimistic light client updates #3499
    • Gossip validation
      libp2p light client gossip validation #3486
  • Light client data collection (server side)
    add option to collect light client data #3474
• Light client LibP2P protocols
  update to pre-release light client sync protocol #3465
  • Propose spec to EF, based on iterations on Lodestar REST API to bring it as close as possible to something compatible with P2P
    Define libp2p protocol for light client sync ethereum/consensus-specs#2802
• REST endpoints to allow light client syncing to latest block header (Lodestar compatible), when a startup flag is specified, and documentation on how to run it
  adopt LC REST API with v0 suffix (without proofs) #3775
  • Update eth2-light-client-demo to Lodestar v0.32
    Update to prater ChainSafe/eth2-light-client-demo#9
    Bump Lodestar deps to 0.32.0, BLS/SSZ to latest ChainSafe/eth2-light-client-demo#12
  • Chronos support for long-running REST handler
    Handle accept() errors properly. nim-chronos#234
  • Helpers to access merkle multiproof generation
    bump nim-ssz-serialization to 3db6cc0f282708aca6c290914488edd832971d61 #3119
    • 64-bit GeneralizedIndex
      Use uint64 for GeneralizedIndex nim-ssz-serialization#13
    • Merkle multiproof generation / Merkle proof for non-root indices
      Add support for building merkle multiproofs nim-ssz-serialization#16
      • Prepare merkleization code for code-reuse with multiproofs / non-root indices
        Reuse chunkedHashTreeRoot for non-basic types nim-ssz-serialization#14
        Cosmetic updates to merkleizer nim-ssz-serialization#12
    • Compile-time optimization support
      allow converting static vars to openArray nim-lang/Nim#19035
    • Merkle multiproof verification
      support merkle multiproof verification #2980
  • Make /eth/v1/config/spec compatible with Lodestar light client
    extend REST config with more spec constants #2962
• EF light client tests
  port EF light client tests #2923