Expose more syncing types to enable custom syncing strategy (#6163)

This PR exposes additional syncing types to facilitate the development
of a custom syncing strategy based on the existing Polkadot syncing
strategy. Specifically, my goal is to isolate the state sync and chain
sync components, allowing the state to be downloaded from the P2P
network without running a full regular Substrate node. I also need to
intercept the state responses during the state sync process.

The newly exposed types are necessary to implement this custom syncing
strategy.