eth: add eth_getHeaderByHash and eth_getHeaderByNumber

[MysticRyuujin]

Implements #874.

Adds eth_getHeaderByHash and eth_getHeaderByNumber. The result is a new Header schema: the consensus header fields plus the derived hash. size and totalDifficulty are excluded, because neither is a header field. The schema is closed, so a client must not add fields. Fields introduced by a fork are omitted for headers that predate it. The result is null for an unknown block and for pending. slotNumber (EIP-7843) is added to Block and Header to keep the schemas aligned.

An unresolvable safe or finalized tag is an unknown block, so the result is null there too. #879 put that rule in BlockTag itself, so this PR no longer repeats it in the method description.

Also adds the testgen generators and 13 geth-generated fixtures.

Client work, one PR per client:

Client	Change	PR
go-ethereum	Return null instead of -32000 for unresolvable tags and pending.	#35627, merged
Nethermind	Return null for pending instead of a header with hash, nonce, and miner nulled.	#13082
reth	Drop size from header responses. The value is the whole-block RLP length. Block responses keep it.	#26912, waits on reth-core#44
Besu	Implement.	#11220
Erigon	Implement in the eth namespace. erigon_getHeaderBy* emits pre-fork fields as null and errors on unknown blocks, so it cannot be aliased directly.	#23717
ethrex	Implement. Pre-fork baseFeePerGas, withdrawalsRoot, and parentBeaconBlockRoot must be omitted, not null.	#7247

Hive rpc-compat against client master images on 2026-09-01, before any of those PRs: geth and Nethermind pass all 13 fixtures byte-identically, reth fails 10 on the extra size field, and Besu, Erigon and ethrex do not implement the methods.

The pending and unresolvable-tag rules are not reproducible on the static test chain, so no fixture covers them.

[magamongo]

Thanks for taking this forward so comprehensively, including the client-side PRs. This matches the direction I had in mind when opening #874.

I am comfortable with the choices to omit size and totalDifficulty, disallow additional properties, return null for pending, and treat unresolvable safe / finalized tags as unknown blocks, subject to #879. Returning null for pending seems cleaner than defining a partially-null header object.