Endpoint to generate an EIP-712 typed data message for authorizing credits usage on EVM chains.

This endpoint is used to retrieve a "CreditsAuthorization" message that the user must sign using their wallet (e.g., MetaMask)
via the eth_signTypedData_v4 method. The resulting signed message (permitCredits) is then supplied to the
/api/redeem/evm/sendGaslessTx endpoint to execute a gasless redeem transaction.

Request Body:

• merchantId (string): The ID of the merchant requesting redemption.
• subtotal (Subtotal): The transaction subtotal (either in USDC or a token-based subtotal).
• transactionData (EvmTransactionData): Transaction details used for calculating gas fees and total credits required.

Response:
Returns an object with the following properties:

• message (string): A JSON string representing the EIP-712 typed data.
• validBefore (string): A timestamp or block number indicating until when the message is valid.
• nonce (string): A unique value to prevent replay attacks.
• creditsRawAmount (number): The raw credits amount calculated based on the transaction subtotal and estimated gas fees.