Expose more syncing types to enable custom syncing strategy

This commit 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
Substrate node. And I need to intercept the state responses during the state
sync process.

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