Introduce helper type for two BlockHashes

[mattsse]

Describe the feature

currently we use an header when we actually only need the number+hash+parenthash

for example:

reth/crates/net/p2p/src/headers/downloader.rs

Line 60 in cef7ec8

Gap(SealedHeader),

with upcoming generics, this would be harder to manage and we can use a new type, like

pub struct BlockAndParent {
  parent: Hash,
  block: BlockNumHash,
}

name tbd

TODO

• introduce the type (can be in reth-primitives-traits first, but we want to move this to alloy later)

Additional context

No response