Consider checking block signatures for column lookup

[dapplion]

In this function

lighthouse/beacon_node/beacon_chain/src/data_availability_checker.rs

Line 228 in 999b045

pub fn put_rpc_custody_columns(

We don't check the block signature. It's a bit sus but may not be an issue. Review if that opens DOS vectors, where an attacker triggers lookups with crappy columns