Improve the definition of the ApiHealthReportEntry class

[fgheysels]

Is your feature request related to a problem? Please describe.

When creating a new project based on the Arcus.WebApi template, we might get compiler warnings because of this line:

 return new ApiHealthReportEntry
            {
                Data = entry.Data.ToDictionary(item => item.Key, item => item.Value),
                Description = entry.Description,  // here we might have compiler warnings
                Duration = entry.Duration,
                Status = entry.Status,
                Tags = entry.Tags
            };

The compiler is issued because it is possible that we assign null on a non-nullable property. Indeed, we see that the Description is declared as a string whereas the Description property of the HealthReportEntry class is declared as a string?: https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.diagnostics.healthchecks.healthreportentry.description?view=dotnet-plat-ext-8.0

I think we should declare the Description property of the ApiHealthReportyEntry as a nullable string as well.