[solana-wallet-snap] Performance improvements

[gantunesr]

1 - The loop inside createAccounts can be parallelized using Promise.all

await Promise.all(indicesToCreate.map(async (groupIndex) => { ... });

2 - Instead of#listAccounts() it could use an "index cache" and avoid deserializing the entire keyringAccounts blob

const keyringAccounts =
  (await this.#state.getKey<UnencryptedStateValue['keyringAccounts']>(
    'keyringAccounts',
  )) ?? {};

for (const account of Object.values(keyringAccounts)) { ... }