feat: add optional createAccounts method based on KeyringV2

[ccharly]

Adding a new optional createAccounts method that we could already leverage today without having the full implementation of KeyringV2.

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Note

Medium Risk
Touches core account-creation pathways and introduces batch mutation with rollback and locking, which could affect state consistency and Snap interactions if edge cases are missed.

Overview
Introduces an optional Keyring.createAccounts method plus a new keyring_createAccounts RPC (KeyringRpcMethod.CreateAccounts) that accepts typed v2 CreateAccountOptions and returns an array of accounts.

Wires the new RPC end-to-end: keyring-snap-sdk dispatches keyring_createAccounts (and errors when unimplemented), keyring-snap-client adds KeyringClient.createAccounts, and SnapKeyring implements batch creation by validating/adding returned accounts to internal state (without per-account notify:accountCreated), enforcing idempotency, rejecting duplicates/unsupported generic accounts, rolling back Snap state via deleteAccount on failure, and serializing concurrent calls via an async-mutex lock.

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[ccharly]

@metamaskbot publish-preview

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.