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

[guuurris]

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

[Totati]

Duplicate of https://github.com/angular/components/issues/17743

[crisbeto]

I think that the title of #17743 is the same, but what it was actually asking for was inheriting the disabled state from the closest fieldset. I think that we can keep this open, but we'll have to do some investigation on whether we can make the accessibility work for this. We could use aria-readonly, but we'll have to check if it makes sense on a combobox.

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[sp90]

This is still an issue today and i believe its very similar to how disable work the difference is essentially if the field is included in the this.form.getValue() which disabled i not but readonly should be.