Fix transfers

[alexex10]

Changes

• Fix sub-accounts transfers settlement (+ example)
• Add HexString type alias to explicitly mark hex string values
• Mask x-api-key header value in logs

[2026-06-22 14:05:26,602 25134 8293851712 DEBUG] Sending GET https://api.starknet.sepolia.extended.exchange/api/v1/info/assets
[2026-06-22 14:05:28,088 25134 8293851712 DEBUG] Sending GET https://api.starknet.sepolia.extended.exchange/api/v1/user/accounts
[2026-06-22 14:05:29,187 25134 8293851712 DEBUG] Sending GET https://api.starknet.sepolia.extended.exchange/api/v1/user/balance
[2026-06-22 14:05:29,461 25134 8293851712 INFO] Transferring 5USDC from `Main account` to `Acc 2`
[2026-06-22 14:05:29,463 25134 8293851712 DEBUG] Sending POST https://api.starknet.sepolia.extended.exchange/api/v1/user/transfer/onchain, headers={<RequestHeader.ACCEPT: 'Accept'>: 'application/json', <RequestHeader.CONTENT_TYPE: 'Content-Type'>: 'application/json', <RequestHeader.USER_AGENT: 'User-Agent'>: 'X10PythonTradingClient/2.1.0', <RequestHeader.API_KEY: 'X-Api-Key'>: '***'}
[2026-06-22 14:05:29,752 25134 8293851712 INFO] Transfer: {
    "valid_signature": true,
    "id": 2068998806774517760,
    "hash_calculated": null,
    "stark_ex_representation": null
}