feat: add remote batch prover

[SantiagoPittella]

Part of #1034

It adds a worker and a client for the RemoteBatchProver, and serialization to related structs. This PR doesn't adds logic to route requests in the proxy to a certain worker type. We might want to have different proxies for each prover type.

Splits logic for servers and clients into new files instead of using mod.rs.

I'm currently looking how to create a ProposedBatch to be used in a test.

[bobbinth]

Thank you! Not a review yet - but one question: could you split out adding of serialization/deserialization logic into a separate PR? (we'll be able to merge it faster and it will also reduce the footprint of this PR).

[bobbinth]

We might want to have different proxies for each prover type.

I haven't really thought this through, but another option could to have workers that can handle different types of proving requests. Basically, have a worker that can accept requests in this format:

service Api {
    rpc Prove(ProveRequest) returns (ProveResponse) {}
}

enum ProofType {
  TRANSACTION = 0;
  BATCH = 1;
  BLOCK = 2;
}

message ProveRequest {
    ProofType proof_type = 1;
    bytes payload = 2;
}

message ProveResponse {
    bytes payload = 1;
}

If this works, then the proxy can be "request-agnostic" - i.e., it would route requests to the next available worker and that worker will be able to handle any type of request.