Replica should forward write requests to stream source

[Mirko-von-Leipzig]

See parent issue for full context.

Node's running in replica mode should forward tx and batch submissions to their block stream source.

This should include doing some basic request verification before forwarding.

• Check reference block's hash, and that it exists.
  • Note: this introduces a possibility for a client desync if they previously queried the block from a different source i.e. this replica might be a block behind and therefore rejects this request. One solution is sticky connections at the load-balancer.
• Check nullifiers don't already exist.
• Check output note's don't already exist.
• Submit to validator.
• .. maybe something I've forgotten..
• forward it

[Mirko-von-Leipzig]

Something I didn't consider is that external replicas won't have access to the validator.

It may make sense to delay the validator portion to the node that actually performs the authentication. In the end-game this would mean the internal replica does this, in the short term this means only the sequencer does this.

For this issue I think the replica should therefore verify the proof locally itself before forwarding.

This issue would also address the addition/moving of stream API to the RPC service (if not done already), as per #2096 sub-item.