feat(button-toggle): should support form field control

[Nosfistis]

Feature Description

The <mat-button-toggle-group> should support being used as a form field control, as it supports @angular/forms. This means that it should implement MatFormFieldControl, as it already implements ControlValueAccessor.

Use Case

I am using <mat-button-toggle-group with <mat-form-field>, however displaying <mat-error> next to it lacks the necessary CSS due to <mat-error> not being nested in <mat-form-field>.