Function polkadot_node_subsystem_util::signing_key_and_index

pub async fn signing_key_and_index(
    validators: &[ValidatorId], 
    keystore: &SyncCryptoStorePtr
) -> Option<(ValidatorId, ValidatorIndex)>

From the given set of validators, find the first key we can sign with, if any, and return it along with the validator index.