feat: make downloaders and clients generic over block parts

[klkvr]

• Makes some of the network types and components generic over NetworkPrimitives This includes EthStream, messages and fetcher
• Adds header and body AT to block clients/downloaders. For now the common implementations mostly rely on alloy_consensus::BlockHeader for header and InMemorySize for body which should allow to reuse those for most of the networks.
• More high-level consumers rely on EthBlockClient which restricts clients to Ethereum block parts

[mattsse]

cool, one step closer.

ci has smol complaints

likely unrelated to this pr