Support for encrypted JSON keystore

[roynalnaruto]

Motivation

This PR adds support for encrypted JSON keystore in the Wallet module in ethers-signers.

Solution

Dependencies added:

1. eth-keystore-rs

API

• Create and store a new encrypted JSON keystore

pub fn new_keystore<P, R, S>(dir: P, rng: &mut R, password: S) -> Result<Wallet, WalletError>
where
  P: AsRef<Path>,
  R: Rng + CryptoRng,
  S: AsRef<[u8]>,

• Decrypt an existing encrypted JSON keystore

pub fn decrypt_keystore<P, S>(keypath: P, password: S) -> Result<Wallet, WalletError>
where
  P: AsRef<Path>,
  S: AsRef<[u8]>,

Tests

Since the eth-keystore-rs crate has its own tests, I've only tested the scenario where the signature generated by a random new keystore should match the signature generated when the same keystore is decrypted.