Add support for light client API specified by the Ethereum specification

[shresthagrawal]

When we started building Kevlar the Light Client API to fetch Sync Protocol information was not specified. But the Lodestar client already had some custom endpoints for Light Clients which were not a part of the Ethereum specification, hence we decided to use them instead of waiting for the Ethereum specification to get finalised.

@etan-status pointed me to the finalised API specification for Light Clients. We should add support for both Libp2p API and REST API.
P2P Interface: https://github.com/ethereum/consensus-specs/blob/dev/specs/altair/light-client/p2p-interface.md
REST API: ethereum/beacon-APIs#247