Giropay integration update

[cmaier]

To re-enable the Giropay (former Paydirekt) as payment method the integration had to be updated according to its documentation.

How to test?

To be able to test the integration a test account in the Giropay sandbox environment is necessary, as well as access to the sandbox app of Giropay and one our apps having Giropay on testing or staging enabled and integrating the changes in this PR.

If the above is true do the following steps:

• Navigate to the screen where a payment method can be added.
• Choose to add giropay
• Steps like the following should be listed that needs to be followed:

• The sandbox should be opened and led through the process
• Afterwards the following screen should be seen in the app:

• ⚠️ It's important to click on the Zurück zu snabble button - do not press back ‼️
• A prompt should appear to unlock the device so that the payment method is added and listed afterwards
• Profit! 🎉

Definition of Done

• Changelog gepflegt
• Dokumentation gepflegt
• Alle Anforderung des Issues sind erfüllt
• Selbst greviewed (aka Self-Reviews (eng))
• Review mit Product Owner (Release-Variante o. Minified Build)

App Tests

• Minified getestet? (aka. Release Build)
• Environments beachtet (Production/Staging)
• Unterstützte Sprachen sind getestet (german seems to be the only supported language)
• Light-/Dark-Mode getestet (Giropay doesn't support dark mode)
• Edge-Cases getestet
• Android API Levels wurden berücksichtigt (minSdk?)

Testing

• Tests geschrieben (aka Unit-Tests, Integration-Tests)
• Tests lokal laufen gelassen

[Fabtron]

Works fine!
Im not sure if we should adjust the wording to giropay since the whole context is giropay, except for the applink and the package name. I cant decide which naming is better since it's fine for methods like that app package, but a mixed wording is also bad.
I'll leave this choice up to u :D

[cmaier]

Renamed new methods and vars to new name Giropay instead of old name Paydirekt.