[Grid] Selection Aggregate does not work when binding GridDataResult

[mbechev]

Describe the bug
When binding the Grid to a GridDataResult object, the selection aggregates aren't calculated. Only count is available.

This limitation comes from the fact that GridDataResult is an object with two properties, whereas the selection aggregates logic relies on arrays.

see selection.aggregate.service.ts file:

    public init(): void {
        if ((this.ctx.grid.selectable as SelectableSettings).cellAggregates) {
            this.data = this.dataChanges.data ? this.dataChanges.data : this.ctx.grid.data;
             ...
        }
    }

this.ctx.grid.data requires one more level deep to access the actual collection (this.ctx.grid.data.data).

To Reproduce
Select a range of cells.
https://stackblitz.com/edit/angular-ygxfpt-nj5g7l?file=src%2Fapp%2Fapp.component.ts

Possibly related to #4201