Preserve provided ApplicationInfo

[pc-coholic]

Description
Adyen's Integration Guide requests that applications provide applicationInformation such as merchantApplication and externalPlatform.

However, this library is overwriting any provided information.

This PR addresses this issue and preserves any existing applicationInformation, only extending it with the adyenLibrary.

Tested scenarios

    def _init_api(self, env=None):
        self.adyen = Adyen.Adyen(
            app_name='pretix',
            xapikey=self.settings.test_api_key if self.event.testmode else self.settings.prod_api_key,
            # API-calls go only to -live in prod - not to -live-au or -live-us like in the frontend.
            platform=env if env else 'test' if self.event.testmode else 'live',
            live_endpoint_prefix=self.settings.prod_prefix
        )

    @property
    def api_kwargs(self):
        kwargs = {
            'merchantAccount': self.settings.test_merchant_account if self.event.testmode
            else self.settings.prod_merchant_account,
            'applicationInfo': {
                'merchantApplication': {
                    'name': 'pretix-adyen',
                    'version': PluginApp.PretixPluginMeta.version,
                },
                'externalPlatform': {
                    'name': 'pretix',
                    'version': __version__,
                    'integrator': settings.PRETIX_INSTANCE_NAME,
                }
            }
        }

        return kwargs

    def execute_refund(self, refund: OrderRefund):
        self._init_api()

        payment_info = refund.payment.info_data

        if not payment_info:
            raise PaymentException(_('No payment information found.'))

        rqdata = {
            'modificationAmount': {
                'value': self._get_amount(refund),
                'currency': self.event.currency,
            },
            'originalReference': payment_info['pspReference'],
            'merchantOrderReference': '{event}-{code}'.format(event=self.event.slug.upper(), code=refund.order.code),
            'reference': '{event}-{code}-R-{payment}'.format(event=self.event.slug.upper(), code=refund.order.code, payment=refund.local_id),
            'shopperStatement': self.statement_descriptor(refund),
            'captureDelayHours': 0,
            **self.api_kwargs
        }

        try:
            result = self.adyen.payment.refund(rqdata)
        except AdyenError as e:
            logger.exception('AdyenError: %s' % str(e))
            return

Fixed issue:
No issue has been filed beforehand

[coveralls]

Coverage increased (+0.5%) to 67.773% when pulling 64bb037 on pc-coholic:applicationInfo into 12cba40 on Adyen:develop.

[KadoBOT]

Thanks for opening this PR @pc-coholic . LGTM.