Rollup node: enrich RPCs for pending outbox messages

What

This MR adds query parameter outbox_level for RPCs /local/outbox/pending/executable and /local/outbox/pending/unexecutable. It also adds a new RPC GET /local/outbox/pending to fetch all known outbox messages with their status.

Why

Useful for the outbox monitor which needs to query outbox levels individually.

Manually testing the MR

For instance, we can fetch an outbox level with

curl -sf http://127.0.0.1:8998/local/outbox/pending?outbox_level=10710672

[{
  "outbox_level": 10710672,
  "messages": [{
    "message_index": 0,
    "message": {
       "transactions": [{
          "destination": "KT1LFTW4o88ZJfvrDLynjmM8Hfc5kTrEiP1F",
          "entrypoint": "withdraw",
          "parameters": ...
        }]
    }
  }],
  "status": "pending"
}]