Manage free header and idempotency/apph

[blackwiz4rd]

The following headers are not yet managed by the library:

• https://developers.satispay.com/reference/idempotency
• x-satispay-apph (e.g. https://developers.satispay.com/reference/create-a-payment)

Do you have plans to integrate it? It would be interesting to let developers add free headers in the future in case you have no time to update the library. Thanks!