We need a way of testing the payment solution locally

### Description

We should have a way of testing the payment flow locally in order to more easily be able to do debugging in frontend. Now you have to publish a frontend version, update a test-app with payment set up, then build and deploy the app before you can test a single code change.

Payment needs the Nets API as well as Azure keyvault in order to perform a process action using a machine user.
We should create a mock of these API calls so that we can consistently test the frontend flow in different scenarios.

### Additional Information

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### Tasks

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### Acceptance Criterias

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No files, tests, or entry points are identified in the issue. Start by tracing the frontend payment flow and its Nets API and Azure Key Vault dependencies, then define how local mocks should represent the different payment scenarios; done means the flow can be debugged locally without publishing or deploying the frontend.

Written by the indexing model from the issue text.