feat(MatSelect): Support for nested select options.

[luisg122]

Feature Description

Support for Material Select Forms to have nested select options.

Use case

Having Select form fields with nested select options.

1. Reference the following image of a Select Form Field, it does not appear to be possible to have nested select options via mat-select. Is this by design, and if so, what were the consequences for potentially supporting nested select options?
2. Though, it does not seem that there are any open or closed feature requests for nested select options.

[ok7sai]

Material Select is an implementation of Listbox pattern which only supports labelled grouped options. The closest thing you are looking for is either Menu or Tree