Update API requests from GET to POST method

[rvdsteege]

@remcotolsma mentioned:

Wij doen volgens mij momenteel altijd een GET request: https://github.com/pronamic/wp-pronamic-pay-pay-nl/blob/62a7b9463b12ed4c098944868e23b87a8fb2085c/src/Client.php

Maar de Pay. documentatie geeft aan dat we voor het opstarten van een betaling/transactie een POST request moeten doen?

https://docs.pay.nl/developers#transaction-start

This PR updates the API request method from GET to POST.

[rvdsteege]

@remcotolsma I've made the changes as discussed at Pronamic HQ:

• updated to API version 13 (checked parameter names etc.; no changes)
• iDEAL issuers are now retrieved from https://rest-api.pay.nl/v13/Transaction/getBanks/json as described at https://docs.pay.nl/developers#transaction-ideal

I also noticed HTTP Basic authentication at https://docs.pay.nl/developers#authentication, but we're using token (40 characters) and serviceId (SL-...) parameters. To update to HTTP Basic auth, we would also need a setting for the account code:

De username is bij een PAY. API altijd waarde van de account code en begint met AT- […]

Question: do we want to add the extra account code gateway setting for HTTP Basic auth and continue to use the current implementation as fallback, so the plugin will continue to work after update?