feat: Add `zks_getBatchPubdata` endpoint

[ilitteri]

Description

This PR adds an endpoint to the RPC that'll let a user like Validium's DA Manager get some batch's pubdata to store in some DA Layer.

How to test

1. Run the hole stack (zk && zk clean --all && zk init)
2. Run the following curl

curl http://localhost:3050  -X POST \
  -H "Content-Type: application/json" \
  --data '{"method":"zks_getBatchPubdata","params":[1],"id":1,"jsonrpc":"2.0"}'

[ilitteri]

@slowli, I solved both suggestions, let me know if there's anything else to do for this PR.

[ilitteri]

Not sure whether CI failure is expected, but AFAIU it's caused by the base branch; the changes in this PR look fine.

Thanks for all the feedback! I'll double-check, make a commit if necessary, and then merge.

[popzxc]

@ilitteri let's keep this on hold until we merge validium mode abstraction. I want us to have a round of discussion on 3rd party DA first -- maybe we'll change the design.