feat(mat-select): Allow values to be displayed that are not included in the options list

[Halanson]

Feature Description

We would like to show a value as "selected" only on the closed mat-select/mat-select-trigger that is not included in the available options list.

Here is an example that tries to achieve this.
https://stackblitz.com/edit/wmjgwt
If you change selectable to true (line 37) of select-overview-example.ts the viewValue and backgroundColor are shown in the closed mat-select as expected. Setting it to false shows only the backgroundColor but not the viewValue. We like to see the viewValue as well.

Use Case

a) Imagine a process A -> B -> C with a strict order and tasks following this process. Now you have a mat-select showing the current process step B and if you open the select box it should only show step C as available option. Currently the closed mat-select shows nothing, the opened mat-select shows step C.

b) Imagine a permission system where the user is allowed to select some options of a select box an some not. Here we could use the disabled option. But to make it easier for the user, not showing the options at all would also be handy.