[solana-wallet-snap] Performance improvements

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

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

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

```ts

const keyringAccounts =
(await this.#state.getKey(
'keyringAccounts',
)) ?? {};

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

```

Locate the solana-wallet-snap entry points for createAccounts and #listAccounts, then inspect how keyringAccounts is stored and deserialized. Compare the account-creation flow with the proposed Promise.all and index-cache approaches, and verify that account creation remains correct while avoiding unnecessary full-blob deserialization.

Written by the indexing model from the issue text.