op-node: Make EL Sync work with sequencer

Sequencer cannot advance blocks when it is in EL Sync mode. Find out why, and use that info for consolidating between `sync.CLSync` mode and `sync.ELSync` mode.

Deprioritized and focusing on the verifier logic.

This is not a requirement for RR Sync Removal. Possibly related with
https://github.com/ethereum-optimism/optimism/blob/262b23187458b6f83a2ddf5fc7d5bd07ff8a7af6/op-node/rollup/engine/engine_update.go#L117-L118
Sequencer may try to block build using `startBuildingBlock` method, and the EL will respond `SYNCING` which makes the sequencer not to advance.