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!

Start by locating the MatFormField/MatInput integration that currently reflects the mat-form-field-disabled class and inspect how the MatInput readonly state is exposed. Confirm the expected behavior for readonly inputs, then ensure MatFormField reflects it with mat-form-field-readonly so its child styling can be customized.

Written by the indexing model from the issue text.