Support delegated proving for operators

[MCarlomagno]

Add operator-side delegated proving support and define the required server/client flow.

[MCarlomagno]

Expected flow:

1. Users build and signs tx
2. send to guardian cosigns
3. guardian generates proof
4. guardian send the transactions and proof to the sequencer
5. guardian responds to the users with success code.

• It simplifies user wallet setup (no need to configure a separate remote prover).
• It makes transaction generation in the wallet really fast.
• It should simplify fee management.

On the last point: the guardian could charge a flat fee for proof generation - e.g., 0.0001 USD. This fee would be paid via a note, and in the world where the guardian also generates batches, the note would be consumed as an unauthenticated note (and so it would be nearly free from the protocol standpoint).