Add block announce validator. (#3346)

* Add `BlockAnnounceValidator` trait.

* Add associated data to block announcement.

* Make tests compile.

* Move validator into `sync.rs`.

* Smaller changes.

* Update core/network/src/protocol.rs

Co-Authored-By: Kian Paimani <[email protected]>

* Update core/network/src/protocol.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update core/network/src/test/sync.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Formatting.

* Remove assoc. data from `BlockImportNotification`.

* Use `Option<Vec<u8>>` for associated data.

* Update core/network/src/protocol/sync.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Fix type error.