In Altair, light client sync protocol exchanges `BeaconBlockHeader`
structures for tracking current progress. Wrapping `BeaconBlockHeader`
inside a `LightClientHeader` allows future extensions of this header,
e.g., to also track `ExecutionPayloadHeader`.
Note: This changes the JSON REST format by adding a `beacon` nesting.
For SSZ, the serialization format stays same (but overall root changes).
Introduce `block_to_light_client_header` helper function to enable
future forks to override it with additional info (e.g., execution),
without having to change the general light client logic.
Likewise, update existing light client data creation flow to use
`block_to_light_client_header` and default-initialize empty fields.
Furthermore, generalize `create_update` helper to streamline test code
using `block_to_light_client_header`.
Note: In Altair spec, LC header is the same as `BeaconBlockHeader`.
however; future forks will extend it with more information.
Future light client protocol extensions may include data from the block
in addition to data from the state, e.g., `ExecutionPayloadHeader`.
To prepare for this, also pass the block to the corresponding functions.
In practice, blocks access is easier than historic state access, meaning
there are no practical limitations due to this change.
Introduces reduced `LightClientUpdate` structures to allow keeping track
of the latest `finalized_header` and `optimistic_header`. This may also
help in scheduling the next query for a full `LightClientUpdate` once
sync committee finality has been reached.
Etan Kissling