Added terminal API endpoints

[iFlyinq]

I've added the terminal API endpoints. Additionally, to facilitate the API's integration into my project, I ensured that the payment's _links include a "dashboard" link. For creating new payments via a terminal, I introduced an optional "terminalId" parameter.

I tried to follow the conventions used across the project, if any changes have to be done feel free to let me know or do them ;)

[iFlyinq]

The endpoint GET https://api.mollie.com/v2/terminals/*id* appears to lack the _links.profile which is typically included, following convention, in other endpoints such as in GET https://docs.mollie.com/reference/v2/subscriptions-api/get-subscription.