Endret response til json for initialize og upload

[mSunberg]

Description

This code changes the response for initialize and upload from text to json.

Related Issue(s)

• #{https://github.com/Altinn/altinn-broker/issues/542}

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

[Ceredron]

Looks like a lot of tests are failing. I think it is because FileTransferId has changed from string to object so everywhere that gets a FileTransferId needs to update. But it should only be a few helper functions. For instance, in FileTransferControllerTests/InitializeAndAssertBasicFileTransfer
It tries to get FileTransferId like this:
return await initializeFileTransferResponse.Content.ReadAsStringAsync();
But should be:

var fileTransferResponse = await initializeFileTransferResponse.Content.ReadFromJsonAsync();
return fileTransferResponse.FileTransferId;