Add HTTP Basic authentication support

[remcotolsma]

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?

Originally posted by @rvdsteege in #4 (comment)