Function validate_state_cert 

pub fn validate_state_cert(
    cert: &LightClientStateUpdateCertificateV2<SeqTypes>,
    stake_table: &HSStakeTable<SeqTypes>,
    expected_epoch: EpochNumber,
    epoch_height: u64,
    upgrade_lock: &UpgradeLock<SeqTypes>,
) -> Result<()>

Validates a state certificate: that it belongs to expected_epoch, and that its signatures reach the threshold under stake_table.