feat(wallets): add idempotency key support to transfer

[warneracw21]

Summary

• Adds idempotency_key to the transfer method input type, matching the pattern already used by rpc and rawSign
• Threads the key through prepareRequest so it's sent as the privy-idempotency-key header

Usage

await privy.wallets().transfer(walletId, {
  source: { asset, amount, chain: sourceChain },
  destination: { address: destAddress, chain: destChain },
  authorization_context: { user_jwts: [jwt] },
  idempotency_key: 'my-unique-key',
});

Test plan

• tsc --noEmit passes
• Verify idempotency header is sent in integration test

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