Readonly CSS Styling: MatFormField should reflect css class 'mat-form-field-readonly' similar to 'mat-form-field-disabled'

#### Please describe the feature you would like to request.

The MatFormField component reflects a disabled MatInput directive via a dynamic css class 'mat-form-field-disabled'.

I suggest to implement a dynamic css class 'mat-form-field-readonly' similar to 'mat-form-field-disabled' which reflects the readonly state of the MatInput directive.

#### What is the use-case or motivation for this proposal?

My proposal would support better css styling. Eg. styling the child 'mat-form-field-underline' of the MatFormField (which is not a dom child of the MatInput directive).

#### Is there anything else we should know?

No special css styling is required to be implemented by you. Only the possibility to implement one by myself.

#### Temporary Workaround

Helper directive (ts):
```
import { Directive, Input, HostBinding } from '@angular/core';
import { MatFormField, MatInput } from '@angular/material';

/* Workaround to provide css class 'mat-form-field-readonly' similiar to 'mat-form-field-disabled'.
* Example Usage:
*
*
*
*/
@Directive({
selector: 'mat-form-field[provideReadonly]',
host: {
'[class.mat-form-field-readonly]': 'isReadonlyInput()'
}
})
export class ProvideMatFormFieldReadonlyDirective {
constructor(private _matFormField: MatFormField) { }

public isReadonlyInput(): boolean {
const ctrl = this._matFormField._control;
if (ctrl instanceof MatInput) {
return ctrl.readonly;
}
return false;
}
}
```

The directive must be assigned explicitly (html):
```

```

Which can be styled for example (css):
```
/* mat-form-field / matInput - readonly support */
.mat-form-field-appearance-legacy.mat-form-field-readonly .mat-form-field-underline {
background-color: rgba(127, 127, 127, 0.25);
}
```

Thank You!

Bắt đầu bằng cách xác định phần tích hợp MatFormField/MatInput hiện đang phản ánh class mat-form-field-disabled và kiểm tra cách trạng thái readonly của MatInput được cung cấp. Xác nhận hành vi mong đợi đối với các input readonly, sau đó đảm bảo rằng MatFormField phản ánh trạng thái này bằng mat-form-field-readonly để có thể tùy chỉnh kiểu dáng của các phần tử con.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.