feat(Material Select): Support for readonly attribute in Material Select component

#### Feature Description
It should be possible to use a **readonly** property similar to have disabled is used with mat-select (Material Select component)

#### Use Case
When re-submitting a form where some select field should not be editable but resent. Property disabled will in this case simple remove selected value from form.

**This is an example that show how it would be used and how it works now:**
https://stackblitz.com/edit/angular-fj6kyz?file=src/app/select-overview-example.html

Start with the Angular Material Select component and reproduce the behavior in the linked StackBlitz example. Implement a readonly state that prevents editing without removing the selected value from a resubmitted form, then verify the form behavior and existing select behavior remain correct.

Written by the indexing model from the issue text.