Add Gloas lightclient api support

Brought to our attention in discussions by @yoshidan in
https://github.com/ChainSafe/lodestar/discussions/9199
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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 https://github.com/ChainSafe/lodestar/discussions/9199#discussioncomment-17229808_

Start by reading the Gloas light-client sync protocol at consensus-specs/specs/gloas/light-client/sync-protocol.md and the linked Lodestar discussion. Then locate Lodestar's light-client API entry points and related tests; done means the API supports the specified Gloas header fields and generalized-index verification.

Written by the indexing model from the issue text.