Re-integrate block producer into block-explorer example

[deekerno]

The producer field has been removed from the Block struct that is returned from fuel-gql-client; we should find a way to re-integrate that information back into the block explorer example.

[deekerno]

We will be able to re-integrate the block producer when the next version of fuel-gql-client is released:

impl Block {
    /// Returns the block producer public key, if any.
    pub fn block_producer(&self) -> Option<fuel_vm::fuel_crypto::PublicKey> {
        let message = self.header.id.clone().into_message();
        match &self.consensus {
            Consensus::Genesis(_) => Some(Default::default()),
            Consensus::PoAConsensus(poa) => {
                let signature = poa.signature.clone().into_signature();
                let producer_pub_key = signature.recover(&message);
                producer_pub_key.ok()
            }
            Consensus::Unknown => None,
        }
    }
}