Make Checkpoint sync work for PeerDAS

[jimmygchen]

Description

Prior to PeerDAS, checkpoint sync requires downloading / providing the checkpoint blob. The rationale is documented here.

From PeerDAS, unless we make all checkpoint servers supernodes AND implement an new /data_columns HTTP endpoint for this, beacon nodes will have to fetch data columns for the checkpoint block via p2p instead of HTTP.

@michaelsproul and @dapplion suggested that we can either briefly allow the invariant to break briefly (storing block first, and then store data columns once they're downloaded) on startup or move the invariant target to the first block after anchor point.

Part of #4983

[dapplion]

@michaelsproul what about changing the invariant to exclude the finalized block?