Switching eFormidling receiver logic based on environment

## Description
When an application is deployed to TT02 there is a different set of receivers compared to an app running in production.
Currently, the eFormidling logic does not support switching receiver based on environment as it is a config value or hardcoded in App.cs

We need a more flexible solution that makes it easier for the app developer to se a receiver based on environment.

## Considerations
- [ ] App owners have signaled that receiver might also vary based on form data.

## Acceptance criteria
- [ ] We have guide lines and documentation available on how to set up receiver of eFormidling shipments from an app.

## Specification tasks
- [ ] Development tasks are defined

## Development tasks
> Add tasks here

## Test
> Add test cases here as checkboxes that are being tested as part of the changes.

## Definition of done
- [ ] Documentation is updated (if relevant)
- [ ] QA

Start by reviewing App.cs and the existing eFormidling configuration to understand how the receiver is currently selected. Define the development and test tasks needed for environment-based selection, accounting for possible form-data variation, and document the setup for app developers; done requires the guide lines and documentation to be available.

Written by the indexing model from the issue text.