# Description

This is a continuation of
https://github.com/paritytech/polkadot-sdk/pull/5666 that finally fixes
https://github.com/paritytech/polkadot-sdk/issues/5333.

This should allow developers to create custom syncing strategies or even
the whole syncing engine if they so desire. It also moved syncing engine
creation and addition of corresponding protocol outside
`build_network_advanced` method, which is something Bastian expressed as
desired in
https://github.com/paritytech/polkadot-sdk/issues/5#issuecomment-1700816458

Here I replaced strategy-specific types and methods in `SyncingStrategy`
trait with generic ones. Specifically `SyncingAction` is now used by all
strategies instead of strategy-specific types with conversions.
`StrategyKey` was an enum with a fixed set of options and now replaced
with an opaque type that strategies create privately and send to upper
layers as an opaque type. Requests and responses are now handled in a
generic way regardl...