feat(l2): prover_server/proposer state in file

[fborello-lambda]

Motivation

Since we need to keep track of each block's zkProof and account updates, a module for creating, reading, and deleting files may be helpful for saving the blocks' state to disk.

Description

Create the module crates/l2/utils/save_prover_state.rs. Which provides the basics to perform read/write operations to files based on the type of information/state we need to store.

At the moment there are two states: Vec<AccountUpdate> and the zkProof.

The structure is the one proposed in Issue #1042:

ethereum_rust_l2/
├── 1
│   ├── account_updates_1.json
│   └── proof_1.json
└── 2
    ├── account_updates_2.json
    └── proof_2.json

The path ethereun_rust_l2 is placed under the directories::ProjectDirs::data_local_dir() (docs).

Closes #1060
Related to #1042