SlotNames behave differently based on column type

**System Information (please complete the following information):**
- OS & Version: Windows 11
- ML.NET Version: 1.7.0
- .NET Version: .NET 6.0

**Describe the bug**

For multiclass classification problems, SlotNames are only available when the columns are of type `string`. Even though, the value and meaning of that value are the same, the SlotName behavior is different based on the data type.

**To Reproduce**
Steps to reproduce the behavior:
1. Train a multiclass classification model
2. Map labels to scores. You can do it with code similar to the following:

```csharp
using Microsoft.ML;
using Microsoft.ML.Data;
using myMLApp;

// Add input data
var sampleData = new SentimentModel.ModelInput()
{
Col0 = "This restaurant was wonderful."
};

// Load model and predict output of sample data
var result = SentimentModel.Predict(sampleData);

// If PredictedLabel is 1, sentiment is "Positive"; otherwise, sentiment is "Negative"
string sentiment = result.PredictedLabel == "1" ? "Positive" : "Negative";
Console.WriteLine($"Text: {sampleData.Col0}\nSentiment: {sentiment}");

var sortedLabels = GetScoresWithLabelsSorted(SentimentModel.PredictEngine.Value.OutputSchema, nameof(result.Score), result.Score);

foreach(var (k,v) in sortedLabels)
{
Console.WriteLine($"{k}: {v}");
}

static Dictionary GetScoresWithLabelsSorted(DataViewSchema schema, string name, float[] scores)
{
Dictionary result = new Dictionary();

var column = schema.GetColumnOrNull(name);

var slotNames = new VBuffer>();
column.Value.GetSlotNames(ref slotNames);
var names = new string[slotNames.Length];
var num = 0;
foreach (var denseValue in slotNames.DenseValues())
{
result.Add(denseValue.ToString(), scores[num++]);
}

return result.OrderByDescending(c => c.Value).ToDictionary(i => i.Key, i => i.Value);
}
```

**Expected behavior**

Not sure? I would think that if the value and "meaning" of that value are the same, the type shouldn't matter and SlotNames are made available.