Introduce `get_lc_beacon_slot` and `get_lc_beacon_root` accessors
similar to `get_current_slot(state)` to account for future extensions
to the light client header structure that may override how those fields
are accessed. Idea is to extend with execution accessors in the future.
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.
