While the current Altair specs define structures to aid light client
development, one missing key aspect is the network protocol definition.

Certain implementations have started defining their own REST based APIs,
e.g., Lodestar at https://github.com/ChainSafe/lodestar/blob/master/packages/api/src/routes/lightclient.ts
While such APIs are useful, REST does not seem to be the ideomatic
choice as the sole API available at such a low level for Ethereum.

This patch introduces a libp2p based protocol to allow light clients to
sync to the latest `BeaconBlockHeader` in a trustless and decentralized
manner, building on top of prior work from:
- @hwwhww at #2267
- @jinfwhuang at #2786
- Lodestar's REST API (also has an endpoint to fetch merkle proofs!)

Co-authored-by: Hsiao-Wei Wang <hsiaowei.eth@gmail.com>