Built-in support for add new item functionality in select-based components

[yanisslav]

Is your feature request related to a problem? Please describe.
At present, we do not have a definitive rendering structure for the MultiSelect popup when the 'Allow custom values' feature is enabled. Although this feature is currently available in both Kendo React and Kendo Angular suites, their implementations differ from each other, and the popup renders in a slightly different manner.

Describe the solution you would like to see implemented
This feature seems to have already been discussed, and there is a design - https://github.com/telerik/web-components-ux/issues/607#issuecomment-1463653343

[yanisslav]

Rendering is also missing for other select-like components, such as the Combobox.

[radkostanev]

Updating the scope of this, as we should ideally add support for this feature in all select-like components, not just one particular we have demand for.

[inikolova]

Based on a research and an offline discussion with @dmanova and @dtopalov, it appears the allowCustom functionality is available only for select elements with inputs - MultiSelect, MultiColumnComboBox, ComboBox and AutoComplete. Therefore I removed the labels for DropDownList, DropDownGrid, DropDownTree.