Dropdown, MultiSelect, Listbox group filter issue when using custom interface instead of SelectItemGroup

[shaifulborhan]

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Plunkr Case (Bug Reports)
Please demonstrate your case at stackblitz by using the issue template below. Issues without a test case have much less possibility to be reviewd in detail and assisted.

https://stackblitz.com/edit/primeng-dropdown-group-filter-demo

Current behavior
When the dropdown is in [group]=true and [filter]=true mode, the filter does not work when we are passing the options data in a format other than SelectItemGroup[] even when the filterBy property has been set explicitly.

Expected behavior
The filter should work as expected when the options are in a data format other than SelectItemGroup[] because we have explicitly define our filter fields in the filterBy property.

What is the motivation / use case for changing the behavior?
It should be working as expected because the component properties are deliberately made to support this functionality.

Please tell us about your environment:
MacOS

• Angular version: 11.X

• PrimeNG version: 11.3.1

• Browser: Tested in Chrome 89 only

• Language: [all | TypeScript 4.x