Support Engine API for POS Single Slot Finality Dev

[mh0lt]

Polygon have a single slot finality project which aim to deliver single slot finality to the POS network by the end of 2025.

To support that development there is a requirement to have a version of erigon which will sync to the tip of Amoy and then change mode so that it relies on the engine api to process block insertions.

A suggested prototypical flow is as follows:

--pos,ssf=true // flag which initiates the flow
--pos.ssf.block=latest|block specified the block to start ssf from (default latest)

If the block is set to latest and a split block has not been previously defined - sync to the tip and wait for the end of the latest milestone and then stop synching.

If the block is passed the split point will be updated to that block and synching will stop there after which the engine API will be utilized.