Reduce Electra boilerplate

[dapplion]

Description

Electra's new attestation type has introduced a match boilerplate.

https://github.com/sigp/lighthouse/blob/c4f2284dbe00ade38e93b1bac35c235386759cb9/consensus/types/src/beacon_block_body.rs#L236-L243

https://github.com/sigp/lighthouse/blob/9a01b6b363d1d54b15b3c37860b1c159eb7b8979/beacon_node/beacon_chain/tests/block_verification.rs#L727-L757

In previous issues, I stressed how we want fork addition to be as easy as possible to foster innovation. Having to add 1000 lines of mindless code just to develop a new feature is not optimal.

We can use some macro magic to prevent a new ForkName::EIP9999 variant from having to modify all these statements.

[michaelsproul]

The example shown could use a superstruct mapping macro

[michaelsproul]

    pub fn attestations_len(self) -> usize {
        map_beacon_block_body_ref!(&'a _, self, |inner, cons| {
            let n = inner.attestations.len();
            cons(inner);
            n
        })
    }

Needs the cons(inner) trick to do type hinting. That's an open issue: https://github.com/sigp/superstruct/issues/31