Introduces additional REST endpoints for light client sync, compatible
with the libp2p specification defined in `ethereum/consensus-specs`:
https://github.com/ethereum/consensus-specs/blob/v1.2.0-rc.3/specs/altair/light-client/p2p-interface.md

Endpoints are defined to be fork-aware to allow potential optimizations
such as including EL block hash into `BeaconBlockHeader` in the future.

The `getLightClientUpdatesByRange` endpoint is a new kind of endpoint,
as it introduces the concept of list responses. Each list item could
potentially be of a different fork. Likewise, for the events stream,
the fork may update between events. Both the list response and the event
are additionall wrapped with a per-item `version` indicator in JSON.
For SSZ, the list items are encoded as length/version/payload triples.
Note that the events endpoint does not support the SSZ format.

This builds on prior work from:
- @dapplion at ethereum#181

The proof endpoint in that PR is out-of-scope here, but could be useful
as part of the debug API as an `eth_getProof` equivalent for CL data.