rpc: drop size from eth_getHeaderBy* responses

[MysticRyuujin]

eth_getHeaderByNumber and eth_getHeaderByHash no longer emit size. The value was the whole-block RLP length, the same number eth_getBlockBy* reports, so it leaked body data into a header-only response. geth, Nethermind, and Erigon's erigon_getHeaderBy* all omit it.

Implements the semantics proposed in ethereum/execution-apis#877 (ethereum/execution-apis#874). Block responses keep size.

Mechanics: RpcTypes::Header gains a SizedHeader bound with a clear_size implementation for alloy_rpc_types_eth::Header, and rpc_block_header clears the field after conversion. An alternative is changing FromConsensusHeader in reth-core to take an optional size; happy to redo it that way if preferred.

Follow-up candidate: a header-only lookup path, since rpc_block_header loads and recovers the full block only to discard the body.

[mattsse]

I think the better solution here would be to change convert_header so that this accepts Optional size or remove that arg entirely, leaning towards removing entirely

[MysticRyuujin]

Both need a reth-core change. FromConsensusHeader::from_consensus_header takes block_size: usize, so the () impl of HeaderConverter cannot build a size-free header today.

convert_header is also the only place block size gets set. Block flattens its header, and into_rpc_block* passes rlp_length to the header builder. Drop the arg and the block path has to set size after conversion, which needs a setter bound on the RPC header type in into_rpc_block*. Same trait as this PR, moved to the block path.

I prefer that version. Size is a block property, so headers never carry it and blocks add it. I can open the reth-core PR that drops the arg from FromConsensusHeader and adds the setter for into_rpc_block*, then rebase this one on the release. If you would rather take the smaller Option<usize> step first, tell me.

[MysticRyuujin]

Opened paradigmxyz/reth-core#44 for the removal. from_consensus_header loses the size argument, and a new SetBlockSize trait lets into_rpc_block* set the field from the block RLP length after the header builder runs.

I have the matching reth side ready locally and checked it against that branch through a temporary [patch.crates-io]. rpc-convert, rpc-eth-api, rpc, rpc-eth-types and the rest of the workspace build, fmt and clippy are clean. RpcTypes::Header gains the SetBlockSize bound, so rpc-convert needs reth-primitives-traits with the rpc-compat feature.

Two notes on that version:

• eth_subscribe newHeads keeps its size field. The refactor drops it by default, so I set it explicitly in header_stream to hold the current output. Tell me if you would rather have newHeads lose it too, which would match the header responses.
• Uncle responses keep size as well, set from the uncle block RLP length as before.

I will push it here once reth-core cuts a release with that change. This PR stays on the current version until then.

[mattsse]

We will finalize this after paradigmxyz/reth-core#44 is merged and included in the next minor reth-core release.

[MysticRyuujin]

Status update. ethereum/execution-apis#877 is out of draft, and so are the Nethermind, Besu, Erigon and ethrex PRs. go-ethereum #35627 is merged.

This PR stays a draft, as agreed. It waits on reth-core#44 and the next minor reth-core release. The replacement version is ready locally and verified against that branch through a temporary [patch.crates-io]. I will push it and take this out of draft once the release lands.