ModelSerialzationService: Add JSON de/serialization based on DataElement.ContentType

[at-bgp]

Description

Add support for JSON content type in ModelSerializationService when de/serializing from/to storage.

Related Issue(s)

• fixes Analysis: Can we change default data format to JSON? And if so, how? #892

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

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[ivarne]

Det var vist en liten ting til som gjensto (beklager dårlig hukommelse, det kan godt være flere)

app-lib-dotnet/src/Altinn.App.Core/Infrastructure/Clients/Storage/DataClient.cs

Lines 150 to 153 in 86cb255

	//TODO: this method does not get enough information to know the content type from the DataType
	// if we start to support more than XML
	var serializedBytes = _modelSerializationService.SerializeToXml(dataToSerialize);
	var contentType = "application/xml";

app-lib-dotnet/src/Altinn.App.Core/Infrastructure/Clients/Storage/DataClient.cs

Lines 235 to 237 in 86cb255

	//TODO: this method does not get enough information to know the content type from the DataType
	// if we start to support more than XML
	return _modelSerializationService.DeserializeXml(bytes, type);