Fix wrong group count if GroupInterval is specified

[JaleChaki]

Bug

DataSourceLoader returns an incorrect group count if the GroupInterval is specified. The code snippet below demonstrates how to reproduce the bug.

var loadResult = await DataSourceLoader.LoadAsync(dbContext.SomeDbSet, new LoadOptions() {
    Group = [
        new GroupingInfo {
            Selector = "DateTimeField",
            GroupInterval = "year",
            IsExpanded = false
        }
    ],
    RemoteGrouping = true, // required
    RequireGroupCount = true,
    Skip = 1 // required
});

// expected: number of group intervals
// actual: number of unique "DateTimeField" values
Console.WriteLine(loadResult.groupCount); 

Fix

The DataSourceExpressionBuilder.BuildGroupCountExpr() method now checks the grouping interval. If a grouping interval is defined, it uses remote grouping expression mechanism instead of SelectCompiler