Store `_pronamic_payment_config_id` in JSON

[remcotolsma]

In https://github.com/pronamic/wp-pronamic-pay/issues/196 i suggested to store more gateway configuration within each payment / subscription. Maybe we can already take this into account within this issue?

payment {
    "config_id": "50"
}

payment {
    "gateway_configuration": {
        "$ref": "http://pay.local/wp-json/pronamic-pay/v1/gateways/configurations/50",
        "post_id": "50"  
    }
}

@rvdsteege ?

[remcotolsma]

In many places we use config_id like this: $gateway = Plugin::get_gateway( $payment->get_config_id() );. We could also consider to change it to $gateway = $payment->get_gateway():

payment {
    "gateway": {
        "$ref": "http://pay.local/wp-json/pronamic-pay/v1/gateways/50",
        "post_id": "50"  
    }
}

[remcotolsma]

Fixed in:

• https://github.com/pronamic/wp-pay-core/commit/67175912057a65d7e9d47cf7e25147dd83a67a27
• https://github.com/pronamic/wp-pay-core/commit/adec7dcd1acab7e0268fad27f1021c390a04b535