Add a few convenience implementations for UpdateDataValues on IInstance interface

[ivarne]

Updating DataValues on an instance requires rather ugly code in DataProcessWrite

   var instanceOwnerPartyId = int.Parse(instance.Id.Split("/")[0]);
   var instanceGuid = Guid.Parse(instance.Id.Split("/")[1]);
   var dataValues = new Dictionary<string, string?>{{key, value}};
   return await _instanceClient.UpdateDataValues(instanceOwnerPartyId, instanceGuid, new DataValues{Values = dataValues});

This PR is intended to simplify the code required, so that it can be written

    return await _instanceClient.UpdateDataValue(instance, "key", value);

Or if you want to update multiple values

    var dataValues = new Dictionary<string, string?>()
    {
        {"key1", value1},
        {"key2", value2},
    }
    return await _instanceClient.UpdateDataValues(instance, dataValues);

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)

[RonnyB71]

@ivarne - starting the new year by reviewing open issues - is this still a change you would like to have? I only had a small comment, but are happy to merge after that.

[ivarne]

Good start of the year ❤️
I updated the code, but instead of new InstanceIdentifier(instance.Id) I added a new constructor so I could send the instance directly.