Use remote batch prover

[SantiagoPittella]

Add support for both local and remote proving for batches.

As a first iteration, we are going to use a enum that holds the prover type in the instance, as Bobbin described in this comment

pub enum BatchProver {
    Local(LocalBatchProver),
    Remote(RemoteBatchProver),
}

impl BatchProver {

    pub fn new_local() -> Self {
        ...
    }

    pub fn new_remote(url: Url) -> Result<Self, BatchProverError> {
        ...
    }

    pub async fn prove(
        &self,
        proposed_batch: ProposedBatch,
    ) -> Result<ProvenBatch, BatchProverError> {
        ...
    }
}