Add Gloas lightclient api support

[matthewkeil]

Brought to our attention in discussions by @yoshidan in
#9199

I noticed that the Gloas light client spec has now been merged into consensus-specs:
https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/light-client/sync-protocol.md

Key changes in the spec:

• LightClientHeader now contains execution_block_hash (Hash32) and execution_branch instead of the full execution payload
• New generalized index (832) for Gloas merkle branch verification

Is there any update on the implementation timeline for Lodestar?

Originally posted by @yoshidan in #9199 (comment)