P2P: handle StateDiffsResponse_DeclaredClass

[kirugan]

We don't handle *spec.StateDiffsResponse_DeclaredClass messages right now. See p2p/sync.go:514

[AnkushinDaniil]

The StateDiffsResponse_DeclaredClass message is necessary because it provides a more secure and reliable way to retrieve class hashes, especially for Cairo 0 classes, where verification without state diffs is impossible. After the spec update it's possible to validate the class hash before downloading large amounts of information. It's still unclear how to use declaredClass in stateDiffResponse.