[Feature] Catch all for extra data in block headers

[onbjerg]

Component

No response

Describe the feature you would like

Foundry has some special handling for some L2s (e.g. Arbitrum), and we use a catch all/fallback field in the block header to look up extra properties on these L2s. This is both used in the forking backend (here) and in Cast (to show all fields in e.g. cast block)

Additional context

No response

[prestwich]

is it sufficient to do something like

struct WithExtraData<T> { 
    #[serde(flatten)]
    inner: T,
    #[serde(flatten)]
    extra: HashMap<String, Value>,
}

[DaniPopes]

RPC types Block has an other: OtherFields field:

alloy/crates/rpc-types/src/eth/block.rs

Line 40 in 42594c2

pub other: OtherFields,

I believe this is done now.