sc-network-test::Peer: block push methods return hashes vec (#12944)

* sc-network-test::Peer: block push methods return hashes vec

This commit reworks the block generation/push methods in
sc-network-test::Peer.

Now methods are providing the vector of hashes that were built.

This allows to get rid of redundant `block_hash_from_id` call, as all
hashes are known just after being built.

Similar approach was taken in BeefyTestNet::generate_blocks_and_sync
method.

This PR is part of BlockId::Number refactoring analysis (paritytech/substrate#11292)

* fix

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

Co-authored-by: Bastian Köcher <[email protected]>